Why did scientists study black body radiations from something as complicated as a hollow container rather than the radiation from something simple like a thin solid cylinder?
A black body is a perfect absorber, and in practice it's difficult to make a material that is a perfect absorber. So to make a black body we choose a material that is as good an absorber as we can find, and form it into a hollow sphere with a small hole in it. The black body is then the hole, not the sphere. It's a nearly perfect absorber because any radiation falling on it is multiply reflected within the sphere. Because the material of the sphere is a good absorber and absorbs most of the radiation at each reflection, the chance that any radiation falling on the opening reflects out again is vanishingly small.
I've never heard of a hollow cylinder being used to model a black body, but the hole at the end of the cylinder would be a black body in a similar way to the hole in the sphere.