Have you had any scans of your prostate and seminal vesicles to check for cancer or any other obvious cause?
Prostate cancer can cause blood in semen - one US hospital reported that 14% of cases of blood in semen referred to them were due to prostate cancer, so it's not a direct correlation.
Most often, the cause of blood in semen is never found, but in the process, all potentially serious causes are ruled out. High blood pressure may be the most common cause.