Silver. "Look there," said Hagrid, "see that stuff shinin' on the ground? Silvery stuff? That's unicorn blood." Chapter 15 - The Forbidden Forest - Harry Potter and the Sorcerer's Stone/ Philosoper's...
Unicorns are mythical creatures that have never really existed. With that fact in mind, an author could give inicorns any kind of blood s/he wanted them to have! For Howrse Quizes it's Silver