Yes I have heard of such a thing, though in relation to Asian babies generally. I think it was called the Mongolian Spot. Not long ago there was an article in a broadsheet (probably the Guardian as I usually buy it) that said that some people whose mixed race babies had this spot at birth had been accused of hitting their children and causing a bruise; of course, a bruise changes colour over time and eventually goes away, whereas this spot stays the same before disappearing at a few months' old.