I personally would go with something like;
This strange black liquid is murky and chalky, and is cold to the touch. When drank, your skin changes to a pallid grey, your lips turn blue and your heart slows to almost nothing leaving you for all intents and purposes a walking corpse. Your type changes to 'Undead' for 1d4+1 hours, you gain resistance to necrotic damage and vulnerability to radiant damage, and you have advantage on persuasion and deception checks against other undead.
So you would appear as a corpse to most living things, and dumb undead would assume you're one of them (with some convincing of both the monster and DM).
