This happened while switching from natural to black PETG:
A closer look:
Those pix happened after trying to extract whatever-it-is with tweezers, so it’s definitely something with a higher melting point than PETG.
Removing the (warm) nozzle with the block held in a vise reveals a tuft of something:
The tuft accumulated several turns while unthreading the nozzle from the hot end.
Heating the nozzle a bit more released the tuft:
The black-to-clear transition tailing off at the bottom came from the PETG around the tuft in the cone-shaped end of the nozzle above the aperture. The 100 mil squares suggest the tuft was a distinct entity, rather than a collection of threads, and might have been over 5 mm long.
Perhaps a fragment of PTFE or another high-melting-point plastic?
Reassemble in reverse order, reset the nozzle to Z=0 on the platform, and it’s all good.