At first, the Heroes struggle to stay on their feet, with the gravity of the black hole threatening to pull them in. Yet after a couple of minutes, they can not only stand, but jump around and fight as normal.
The gravitational pull of the black hole is so intense that it sucks out the weapons of the Heroes' hands, yet when they are inside the hole, they can freely jump around from wall to wall, without the black hole's center pulling them deeper. In fact the physics of the black hole seemingly change from scene to scene.
A black hole is not an actual hole whose wall can be climbed, as this episode seems to suggest. It's really a spherical object with an immense gravitational pull.