Thanks! The black hole was inspired by this shader: https://www.shadertoy.com/view/tsBXW3
It uses sphere tracing to simulate 3d rays that bend around the black hole until they hit the accretion disk or the hole itself. The rays that escape are projected back to the 2d viewport and give the distortion effect. My version is simplified since I didn't need as much detail for a pixel art game and I wanted to make sure it didn't slow the game down. I added some constraints to limit the distortion to a certain radius, otherwise you could see reflections of the hero and the boss around the hole and the whole level was slightly distorted and looked a bit jarring. For the transition I just played around with the hole radius, bend force and accretion disk angle. The exit transition uses a similar shader with no accretion disk or black hole, only distorting the level.