Hello,
Thank you for your query at DoctorSpring.com
Blood in the semen can happen without any reason and once it happens it shall come for 6-8 weeks till all blood clears from blood.
However it can happen because of high blood pressure also and you should get your blood pressure checked for few times. If it comes high then you shall need treatment for the same.
Take care
Sincerely
Rajiv Goel