Blood in the semen, or hematospermia, is not caused by a varicocele. A varicocele is an enlargment of the veins that run around the tubes that conduct semen; however there is no physical connection between these veins and the tubes that conduct the semen, so there would be no way for blood to leak into the semen from the varicocele.
There are, however, several causes of blood in the semen that should be investigated.
Probably the most common cause of hematospermia is a sexually transmitted infection, caused by chlamydia, gonorrhea, or another organism. Often there is also pain or burning with urination. These infections can be ruled out by simple tests your doctor
Probably the next most common cause is an inflammation of the prostate. This can be acute (often from the same organisms that cause the sexually transmitted infection) or it can be chronic, when it manifests as low level aching pain in the pelvis and some urinary symptoms. Chronic prostatitis is more common in those who have to hold their urine for a long time, like long distance truck drivers.
Hematospermia is a symptom that should be examined by your primary care doctor
as soon as possible, to make sure one of these infections is not going on.