I don't think its animal blood, fake blood, etc. Thats a TV hokey ploy. Whoever did this wasnt kidding around or trying to frighten anyone. He was there to nab Holly and not leave potential evidence or anything behind.
So... the blood is hers (would not tell us anything) or his (maybe he got cut, bit, scratched) in the assault. In that case LE can do several things. THey would look for a match in the offender data base. If the suspect had submitted a DNA sample for some sort of prior he would be in the system. OR LE could, possibly, try to obtain DNA samples from a possible suspect. i.e. the cops suspected someone they might try to get a DNA sample to see if it matched the blood.