OK don't laugh, GayHealth.com is an awesome site to check out when you have men's health questions. I've found a couple of links that deal with blood in your semen.
http://www.gayhealth.com/templates/1...BLOOD,IN,SEMEN
http://www.gayhealth.com/templates/1...BLOOD,IN,SEMEN
http://www.gayhealth.com/templates/1...BLOOD,IN,SEMEN
Hope these will put your mind to ease a little but I would still go see a doctor. It never hurts to get a checkup.