The description for a Soulknife's psychic blades says,
Also at 3rd level, You can manifest your psionic power as shimmering blades of psychic energy.
Are there similarly described items that have more information in terms of how much illumination they provide?
Are there other rules/sources of guidance in terms of whether a psychic blade would give away a soulknife that was attempting to hide or move stealthily, or was invisible (etc.)?
When an object, a spell or any other game feature emits light this is specified in the description, among which "type" (bright or dim) it sheds.
The Flame Blade spell description says:
The flaming blade sheds bright light in a 10-foot radius and dim light for an additional 10 feet.
The Crystal Blade has the following property:
While you’re holding the sword, you can use a bonus action to cause it to shed bright light in a 30-foot radius and dim light for an additional 30 feet, to cause it to shed dim light in a 10-foot radius, or to douse the light.
Twilight and (surprise surprise...) Light domains present some features that use light, and when these have mechanical consequences they are specified:
Corona of Light. Starting at 17th level, you can use your action to activate an aura of sunlight that lasts for 1 minute or until you dismiss it using another action. You emit bright light in a 60-foot radius and dim light 30 feet beyond that.
Twilight Sanctuary. As an action, you present your holy symbol, and a sphere of twilight emanates from you. The sphere is centered on you, has a 30-foot radius, and is filled with dim light.
The quoted text of Psychic Blades refers to shimmering blade: looking at the definition in MacMillan dictionary one can read:
to reflect a gentle light that seems to shake slightly
But the example below is more explanatory, to me:
The desert landscape shimmered in the midday heat.
In my opinion, the authors of this feature poorly chose the wording1 and they intended that the blade is translucent, is made of psychic force and blurs the sight of what is beyond the weapon.
From a "class point" of view, it could be very strange that a Rogue subclass possesses an ability that could easily give away their stealth.
1 What a surprise, again.
There are no secret rules. The rules don’t say it emits usable light, so it doesn’t.
Things can shimmer in reflected light.
Shimmering does not necessarily mean illumination when used in dnd descriptions. I could find no explicit connection between the use of "shimmering" and emitting light when searching for the word on DndBeyond.
Cambridge Dictionary gives us this definition for shimmering:
reflecting a gentle light that seems to move slightly
Reflecting would mean that the blades are not the source of the light.
So, like everyone else stated, since the rules don't say they produce light, then they don't produce light. However, there is always the wiggle room of DM saying differently.
If you read the rest of the feature:
Whenever you take the Attack action, you can manifest a psychic blade from your free hand and make the attack with that blade. This magic blade is a simple melee weapon with the finesse and thrown properties. It has a normal range of 60 feet and no long range, and on a hit, it deals psychic damage equal to 1d6 plus the ability modifier you used for the attack roll. The blade vanishes immediately after it hits or misses its target, and it leaves no mark on its target if it deals damage.
The blades are not something the rogue would hide with, walk around with, etc. They appear when you attack and disappear when the attack is finished. So if your rogue was hiding, they would would leave hiding to make the attack and then the blades would appear. If they were invisible, the invisibility would drop when they started the Attack action at which point the blades would be summoned (unless greater invisibility, but you understand the context).
So you can dramatically state that the blades come out with a flash of light, but there is no light until you take an Attack action.
