The darkening of the semen may be caused by the presence of blood in the semen. Normal semen has a viscous jelly-like appearance, white or milky white in color, and opaque, and the semen will begin to liquefy after five minutes. If the semen is yellowish, it may be due to too much time of abstinence without discharge or due to prostatitis; If the semen appears dark red or rusty, blackish, it may be hematospermia, which is most commonly caused by seminal vesiculitis, and may also be caused by seminal vesicle tumors. Regardless of the cause, as long as the semen is black or red, then you should go to the regular hospital at the first time to find a professional doctor to find out the cause, and to carry out the most suitable treatment, so as not to delay the condition.