Black homes are so massive and heavy that they bend light (well, technically every piece of mass does, even yo momma). It’s bend so extreme that that accretion disk appears warped.

A similar thing happens with neutron stars that can also bend light in such a way that you can actually see part of the back of the star (if you were able to see it anyway, it would be dark) as light that would be emitted from parts of the back would warp around to the front where you could see it