Doppelganger
A doppleganger's blood is a dark red, almost black, fluid with a low viscosity and a rainbow Sheen . When the vial is held, the blood changes color to match the blood of the creature holding it.
I really do try to make most of this original, but since Forgotten Realms is an integral part of the world I can't help if anything is used without formal permission, but is still "fair use" as I am just using this for my private games.