I've never seen evidence of any free-floating obstructions in the lines that actually plug up as a result of normal bleeding, and can't think of how such a thing would actually be created. In my experience it's usually just common corrosion inside the caliper, the result of time, water, chemical breakdown of the brake fluid, and a lot of heat. When this is happened I've always been able to clear out the obstruction in the caliper mechanically with a probe. You don't have to poke in very far; it's usually right around where the bleeder screw is. It feels like a hard little crust, and when you poke through, it breaks up and then the fluid starts to flow just fine. Especially if the MC wasn't new, I'd feel nervous about nudging the pressure up in the hopes of blasting this kind of corrosion out.