I was giving my boyfriend oral sex and when he ejaculated it was blood. what caused this to happ?

Hematospermia: He should see a urologist. It could be his prostate or something else. It can be benign or serious, but it isn't really a normal thing to have happen.

Hematospermia: Most likely, we'll never know why it happened. If he had pain, perhaps trauma (overenthusiastic?) or infection (prostatitis). Check out http://www.Mayoclinic.Com/health/blood-in-semen/my01214. If he's of similar age, there may be slightly increased risk for prostate cancer, thus should be checked out but data is sparse. Also see http://www.Uptodate.Com/contents/hematospermia & http://goo.Gl/hrpysv.