Occult blood in the semen (or urine for that matter) deserves further attention. The prostate would be the most common source of blood in someone your age. Other causes include infection, bladder or kidney diseases.
I would suggest a transrectal ultrasound to evaluate the prostate as a reasonable first step. If negative, you can consider a cystoscopy to comprehensively evaluate the bladder. If the PSA is in an indeterminate range, you may want to consider obtaining a "free PSA" level, which can suggest malignancy if the levels are low.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Thanks,
Kevin, M.D.