Astronomy

Depending on the mass of the black hole, the "shell" doesn't need to be a shell it could be effectively completely solid with an atom sized black hole at the centre.

PBH's as discussed in this article have pretty wild mass ranges, so anything is possible. It's entirely possible to have black holes so small they can't easily absorb new matter as they're smaller than protons. Tiny black holes only have large surface gravity, nothing noteworthy at a distance.