Blood in the semen is often from a benign process such as a prostatic cyst or stone that occludes outflow of semen from your ejaculatory ducts. Your seminal vesicles produce most of the fluid that comes out with ejaculation. These travel into the urethra (the tube that you pee out of) via ejaculatory ducts. Partial or complete blockage can lead to blood in the semen. This typically resolves with time.
If often resolves on its own but one episode of bleeding into the seminal fluid often takes a few weeks to clear.
If it persists, this can be evaluated with a transrectal ultrasound (ultrasound probe in your bottom) to see what the cause is; they can also put you to sleep and place a catheter in your penis and study the ejaculatory ducts as they empty into the urethra via a xray study. If it is persistent they can resect the prostate endoscopically to help prevent further bleeding but this is not always successful.
Best of luck,Dr Patel