Michael Schirber at Space.com:
The simple answer is “black” because light cannot escape the gravitational pull of a black hole. But light traveling just outside a black hole will be bent — similar to what happens in a lens.
Kornreich and his student Bryant Gipson have figured out how images of landscapes and planets would be distorted by having a black hole sitting in the foreground.
Such mathematical calculations have been done before for stationary black holes, but this is the first time it has been done for spinning black holes, Kornreich told SPACE.com. Most black holes in the universe are thought to be rotating — many at high speeds.
In a stellar black hole, which forms when a giant star dies explosively, the rotation is a logical remnant of the star’s spin. Just as a skater speeds up when she pulls her arms in, the dead star’s rotation picks up dramatically as remaining material collapses into a small, dense black hole.