If black holes are formed during supernovas, how does material get expelled from the supernova?
A supernova is actually the combination of several events including an explosion and a collapse.
When a star goes supernova, a fusion process starts which causes a significant amount of the mass of the star to undergo fusion in a short period of time. This results in a massive explosion which causes the outer layers of the star to expand out at high speed.
The core of the star however is unable to sustain fusion reactions to stop its collapse under gravity. If the core Swiss massive enough it will collapse into a black hole.
A black hole is simply a massive, but relatively small object operating under gravity. It can only consume material which falls into it. The material ejected in the supernova explosion is too far away and traveling far too fast for the black hole's gravity to capture it.