If the blower wheel was jammed up by the broken plastic pieces and preventing the motor from turning, it could easily blow the fuse. So you have likely found the problem.
Maybe I'm reading this wrong, but just to be sure... Am I reading correctly that you have replaced 15 amp fuses with 20 amp fuses? If so, please change them back to 15 amps immediately! The wire is rated for 15 amps and therefore must be protected by a 15 amp fuse. A 20 amp fuse could allow 20 amps to flow continuously in 15 amp wire, causing it to overheat and start a fire. The 15 amp fuse already popped and saved the blower motor from burning out, and possibly a fire as well (as it was designed to do).