How does the black hole at the center of M87 look like in the visible spectrum? I learned here that the picture of the black hole we saw all over the news was a false color image, based on the radio spectrum emissions. How would the black hole at the center of M87 look like when seen by the naked eye from a close enough distance, or by a large enough optical telescope? (please tell me if the two would be different)
Disclaimer: this is speculative, since no images of a black hole have been taken with enough resolution in visible light.

The black hole represented in the movie Interstellar is moderately realistic with the knowledge we have at present. In fact, the movie makers asked astrophysicists to contribute. 

From the article Gravitational lensing by spinning black holes in astrophysics, and in the movie Interstellar, this is a moderately realistic view of a black hole. 



The light is not the black hole itself, but the accretion disk around it. The circle of light is the part of the accretion disk that is behind the black hole, the light of which is lensed by the black hole. 

Now for a black hole spinning on itself, then it would not look as symmetrical. Doppler shifting would make one side appear more red, and the other more blue.