Hi,
Blood is semen is called hematospermia which is usually a benign condition and can be harmless. However it could also be due to Infections,
prostate conditions, urethera abnormalities, lesions in seminal vesicles or systemic conditions such as hypertension etc.
If it is persistent, then you should consult a doctor for a complete evaluation.
Take Care and do write in if you need more information.