I have black vein on my left side of scrotum is it varicocele?

Varicocele. Varicoceles are tortuous veins around the testicle (mostly on the left side). They are deep inside the scrotum and not usually visible unless they are high grade (grade 3-4). If the vessel is on the surface skin of the scrotum, then that is not a varicocele. A simple physical exam and sometime ultrasound of the testicles can diagnose a varicocele.
Unlikely. Varicoceles are inside the scrotum and are typically not visible. They are usually found by feel and not by appearance. It is possible to have a varicocele and a black vein on your scrotum but many times veins like you are talking about appear without an associated varicocele. If you are concerned that you might have a varicocele, you should see your doctor.