Human Blood is a potion that can be found in Vicente Valtieri's room of the Dark Brotherhood Sanctuary. It can also be found in Castle Skingrad, by the Pale Lady in castle's wine cellar, and in the last room of Crowhaven (the fort in the "Origin of the Gray Prince" quest). Note that unless one has the appropriate updates, this potion doesn't "satisfy the thirst" of a vampire. Strangely, the bottle has the same in-game appearance as that of wine.