There are at least two or three kinds of black sand. I have more of a beach sand type in my nano reef, and it is magnetic. It has been running for 5 years, and no problems I'm aware of.
In my seahorse tank, I have a combination of "IndoPacific Black" (which is a combination of black and white), and Tahitian Moon. That looks more like a volcanic sand, and doesn't appear to be magnetic.