What is the shape of a black hole?
There is no true shape.
Black holes are an infinitely small object themselves, being smaller than the size of an atom. Which in turn means they have no shape. However, the area their gravity affects is round because they pull equally in all directions.
Black holes are spherical.
A black hole is defined by its event horizon. This is defined to be the Schwarzschild radius
Nothing can escape the event horizon.
This means that the black hole is spherical.
The Schwarzschild solution to Einstein's field equations assumes that the black hole is not spinning. If it is it will be a slightly flattened sphere in shape.