Interstellar is a groundbreaking film, mostly for its correct use of physics. The physics of this film are also pretty hard to understand if you're not a physics person like me. I'm going to try my best to analyze black holes, specifically the black hole in the other galaxy, Gargantua.
Gargantua is a black hole which is at the center of the galaxy that Cooper and his team are trying to find a viable planet to reestablish the human race on. Gargantua is a massive black hole, and Miller's planet is as close as it could be without being pulled apart and destroyed. Every hour on Miller's planet is seven years back on Earth. Kip figures out that Gargantua must have the mass of 100 million suns. The circumference of a black hole is related to the mass by the Schwarzschild Radius.
You use this formula once you've found the black hole's circumference. Then using C=2πR it gives you R, then you can find out the black hole's mass. Kip has suggested that Gargantua's radius would be 150 million kilometers.
Black holes do not emit any light, so the only way to see them is to interpret the way light behaves around it. "Gargantua casts a black shadow on the field of stars and it also deflects the light rays from each star, distorting the stellar pattern that the camera sees". The stars move at very high speeds so they look as if they are moving very fast in some spots, others seem like they are just floating, and some seem frozen. Gargantua must be a very fast spinning black hole in order for the crew to have such a drastic time loss when they get very close to it.
Thats about all that I understand from the physics of the movie, sorry if it sucks.