when silver nitrate comes in contact with human skin it almost instantly stains the skin black for quite a while. In the part of the movie where the cop/vampire friend visits the black lady in her apartment (after being given the vampire mace made from garlic and silver nitrate)she sprays him in the face with the mace. his face doesn't turn black.Later in the movie his face still isn't black. before people comment on this i stress the stain cannot be washed off unless the substance in rinsed off as soon as it touches the skin, and we see he has the mace on him for plenty of time for it to have stained.