|It turns out that the recipe isn't symbolic, but absolutely literal.|
A basic alchemical distillation from a very old recipe. Widespread throughout texts and traditions, it is believed to be a necessary step on the quest to extend life and cheat death. It appears to be an opaque black liquid with the consistency of milk. It tastes inky and metallic, and remains cooler than room temperature.
Once the Dark Sun has risen on the Astral Plane, quaffing from the Black Flask (which does not detect as magic) has the following effects:
First Dose: For 10 minutes (1 turn), you are under the effect of a blur spell. This is does not detect as magic.
Second Dose: For 10 minutes (1 turn), all damage you give and receive is halved but you get an extra attack as your shadow takes on some of your substance and volition. This does not detect as magic.
You must make a Powers Check.
Third Dose: You dematerialise/turn Ethereal and everything held/worn drops to the floor. You can only detected by those in a similar state or by true seeing. Those with second sight will be aware of your presence, but no more.
For 30 minutes (3 turns), you are naked, insubstantial, invisible and unable to interact with the material world, but free do what you can/wish in your new state. You are apparent to any other entities in a similar state, native or otherwise.
You can feel the pull of the Dark Sun even if you do not recognise it. You perceive shadows and reflective surfaces on the material plane as absolute liquid darkness, in contrast to the hazy greys of everything else.
You must make a Powers Check, remaining immaterial if you fail.
Until such time as the Dark Sun rises on the Astral Plane, the Black Flask is merely an otherwise hollow ritual of alchemical research with the following effects:
Obtaining the Black Flask: researching and creating the Black Flask requires a Powers Check, and the resulting potion detects as magical.
Taking a Dose: Save or be nauseated for d6 x10 minutes (not cumulative with further doses).
You experience the effects above as standard drug-based hallucinations and come away disappointed, or determined to try again.
You are in an Altered State for the durations given above (no save).
During this time, you will have the sense of being stared at by invisible and/or astral/ethereal creatures, if any happen to be present. Astral/ethereal creatures can be confident that you cannot actually see them; those in the material world might believe they have been detected.
All credit for the original to u/the_pint_is_the_bowl, posting in r/DnDBehindTheScreen.
Powers Checks are from Ravenloft (2e AD&D setting), specifically the Masque of the Red Death supplement.
I've got a particular setting in mind for this, and the original slotted straight in like there was already a space for it.