I have a blue birthmark on my wrist, what does that mean?

So, im chinese, and i searched up "blue birthmark" and it came up on wkikpedia that its called a mongolian blue birthmark, But it says that its most commonly found on the lower back or and mines on my wrist, directly on top, and it a dark-blue shade.. I've had many people even ask if i had a bruise, tatoo, or ink. Is it heredditory? and if it, i dont know if anyone in my family tree has had it before. What could it mean? And maybe any other useful info would help, Thanks.