The modern Black Hebrew Israelite movement is definitely a fringe religion - it was started in the late 1800's when a couple of people claimed to have received mystic revelations that African Americans are the direct descendants of the Hebrews. This is not an idea that's backed up by any genealogy or cultural research; just a vision. From that idea, several different groups and churches began to spawn - many of which are considered offshoots of Christianity by religious scholars and other religious groups, because they often involve Christian doctrine and scriptures that most other groups of Jews reject.

There's a current extremist fringe among some of those groups with especially harmful beliefs (anti-Semitism, black supremacy, violence against other groups, etc.) but there are also many Hebrew Israelite groups that are much more moderate - they don't advocate violence or racism. It's important not to lump everyone who's a part of the religion as a whole (even if not all of us agree with the premise of the religion) with the actual extremists that pop up on the news.

Outside of those groups, there are African Americans who are Jewish - traditionally Jewish, not Black Israelite. There are many Jews of color around the world, though there's some disagreement over whether "Jews of color" is a useful umbrella term or not, since it lumps together a huge number of groups.