Hfrhyu

Unlike the other planes, the positive and negative planes are notoriously sparsely described. In fact, I figured that it wasn't even possible to go there until recently, when I read the description of the Wand of Orcus in the DMG:

Destroying the Wand. Destroying the Wand of Orcus requires that it be taken to the Positive Energy Plane by the ancient hero whose skull surmounts it.

I believe it's unlikely that they made a typo here or some kind of other mistake. Therefore, it is possible to travel to the positive (and presumably also the negative) plane.

This raises the question what the plane looks like.

A quick Google search yields results that describe it as an endless expanse of blinding, healing light that unfortunately is too much to handle for mortals and obliterates them.

Is this description accurate?

Since I'm pretty sure that there's no further description in 5e (correct me if I'm wrong), I'm primarily interested in lore from previous editions, but for use with 5th edition.

For completeness, here are the descriptions from the PHB and DMG:

PHB:

Positive and Negative Planes

Like a dome above the other planes, the Positive Plane is the source of radiant energy and the raw life force that suffuses all living beings, from the puny to the sublime. Its dark reflection is the Negative Plane, the source of necrotic energy that destroys the living and animates the undead.

DMG:

The Positive and Negative Planes. These two planes enfold the rest of the cosmology, providing the raw forces of life and death that underlie the rest of
existence in the multiverse.

They may not "look" like anything: they are energy sumps.

This answer is informed by the first edition AD&D descriptions of the Positive and Negative Material planes in the PHB and DMG. I reach that far back because following editions tended to build on that, though as usual editions vary. (For some 2e and 3e perspectives, see @QuadraticWizard's answer here).

1. 
   

   Those two planes power spells (AD&D 1e DMG, p. 40)

   
   
   

   
   

   All magic and cleric spells are similar in that the word sounds, when
   combined into whatever patterns are applicable, are charged with
   energy from the Positive or Negative Material Plane. When uttered,
   these sounds cause the release of this energy, which in turn triggers
   a set reaction. The release of the energy contained in these words is
   what causes the spell to be forgotten or the writing to disappear from
   the surface upon which it is written. When spell energy is released,
   it usually flows to the Prime Material from the Positive or Negative
   Material Plane. To replace it, something must flow back in reverse.
   The dissolution and destruction of material components provides the
   energy that balances out this flow, through the principle of
   similarity. Sometimes this destruction is very slow, as is the case
   with druids‘ mistletoe. Those spells without apparent material
   components are actually utilizing the air exhaled by the magic-user in
   the utterance of the spell.

   
   

   
   


2. 
   

   They are less a "place" than a "state of being" for practical
   purposes. (AD&D 1e DMG p. 112)

   
   
   

   
   

   Deities will not intervene on the planes which are the habitation of
   other deities, i.e., the Outer Planes. They will neither venture to
   involve themselves in the Positive and Negative Material Planes.

   
   

   
   
   

   As Gertrude Stein might say, "there is no there there" to the extent that deities (who can travel most anywhere) just don't go there.

   
   


3. 
   

   Describing what anti-matter looks like is no small task, if it can be done at all(thanks, @Molot).

   
   
   

   
   

   The Positive Material Plane is a place of energy and light, the place
   which is the source of much that is vital and active, the power supply
   for good. The Negative Material Plane is the place of anti-matter and
   negative force, the source of power for undead, the energy area from
   which evil grows. (AD&D 1e PHB, p. 120)

   
   

   
   
   

   As to the 5e Positive plane from the question,

   
   
   

   
   

   ... the Positive Plane is the source of radiant energy and the raw
   life force that suffuses all living beings, from the puny to the
   sublime.

   
   

   
   
   

   Your initial instinct in describing the positive material plane seems to be spot on. Pure light, pure energy, may overwhelm all visual senses - so how does it look? That's very difficult to describe. My estimate is "stand next to the sun with no sunglasses on (with heat protection) and describe what you see." It is such pure light/energy that it overwhelms mere mortal senses and even deities don't go there. The negative material plane is described as being its opposite. An implication of this is that they transcend the physicality of all of the other planes.

   
   
   

   They way I've always seen it played, when they came up, is that those two planes are a physical manifestation of the Yin and Yang¹. The two planes aren't so much physical as metaphysical. In trying to describe those two planes, one might as well try to describe "What does happiness look like? What does sadness look like?"

   
   

The material that became the Planes treatments in the PHB and DMG in AD&D 1e were broached in Dragon Magazine 8, which went into such esoterica as life stealing swords having a direct linkage to the Negative Material plane. In typical Gygaxian brainstorming fashion, we read ...

If it is accepted that the reason that certain creatures can only be
hit by magical weaponry is because the creature exists in two or more
planes simultaneously, then it follows that the weapon must likewise
extend into the planes in which the creature exists. (Dragon 8, p. 4)

But nothing to describe either the Positive or Negative plane beyond sources of power/energy/magic.

Len Lakofka, in Dragon Magazine #42 (page 24-30) went to great lengths to try and create a way to "play on the inner planes" which ran into 7 pages of dense text and tables, tables, tables. It was too clunky for any of our gaming groups. How things "looked" didn't rank very highly in importance in that article.

In Dragon 73, Gygax expresses that the lack of clarity had finally gotten to him, so he put together another article on how the planes worked.

Isn't it interesting to note how the Positive Material Plane sits upon
the material multiverse as if it were a plate? Observe also how the
Negative one serves as a saucer for the same body? (Dragon 73, p. 10)

What the Pos and Neg looked like, when all other planes were given a color in that article (Ethereal purple, Concordant Opposition, Brown) was left unclear.

¹_{If I ever find the article that first gave us that idea, I'll add a citation. Might be an old Dragon magazine. It might also be some old Planescape material that I no longer possess.}

The Positive Plane is an overpowering expanse of white light

Although D&D 5e's DMG has only limited information on these planes, the D&D 3e adventure module Bastion of Broken Souls (2002), which takes place on the Positive Energy Plane, describes it thus on pages 19-20:

This is a place of brilliant white. Pulsating brilliance bleaches out the spectrum and creates stark shadows. Anything more than 5 feat away becomes an indistinct blot of darkness moving against the white background—the blots nearby are certainly your companions. Distance is impossible to gauge within this brilliant environment.

And:

An important facet of this knowledge reveals that living creatures do not last long before swelling and burst with excess life ... The Positive Energy Plane is like the heart of a star. It is a continual furnace of creation, a domain of brilliance beyond the ability of mortals to fully comprehend. Its very being wavers and ripples as new matter and energy is born.

The plane is a wide open undifferentiated expanse. The light is so blindingly bright that even tying a cloth blindfold over one's eyes, it is possible to see to a distance of 300 feet. Simply existing on the plane heals creatures, but this will soon overheal them to the point that they are annihilated by the energy.

The plane is not coexistent with the Ethereal Plane. Gravity is subjective; you fall down whichever way you currently imagine to be down, allowing adventurers to "fly" around with some practice.

Parts of the plane's energy have actually solidified into crystals which are invulnerable to any magic short of direct divine intervention. Entire structures of this form exist. One such location, a ten mile diameter hollow fractal crystal inhabited by numerous creatures, is described as follows:

Spans of crystal, miles long and hundreds of feet wide, pulse with a languide cadence of brilliance. The spans from the edges of what resembles, for want of a better comparison, a snowlake of titanic proportions. Only the edges are apparent; the bulk of the structure is lost in the glare. Specks of darkness congregate near the nexus of several crystal spans.

Interesting side-note: you can actually see a map of part of this structure in the finale of Buffy the Vampire Slayer, where they play a tabletop RPG and a map from this adventure module is used as a prop.

AD&D 2e Planescape sourcebook The Inner Planes (1998) describes that it's very difficult to actually get to the Positive Energy Plane. Many plane-shifting magic items are specifically built with a safeguard to stop the wearer traveling to the Positive Plane, since it's dangerous to travel there and there are very few good reasons to do so.

There's no air on the plane per se, but the life-giving healing energy of the plane sustains visitors without air, food or water.

The Negative Plane is an infinite black void

The Inner Planes describes the Negative Energy Plane as a limitless expanse of destructive, annihilation energy.

It is completely black, absolutely dark. Even sound does not travel far, and the sense of touch is itself dulled. Items conjured into the plane crumbles within one round, and living creatures suffer constant damage. Navigation is nearly impossible.

There is no air to breathe. The only creatures which live here are incorporeal undead, who thrive in the negative energy environment, though physical undead are soon annihilated by the plane's energy.

You can find a lot of information about the Positive and Negative Planes in the Planescape supplement The Inner Planes. It can be purchased as a PDF from DMsGuild.