black residue?

[tmcninch]

A few frets have this hard, black residue on one side or the other (see photos). Is this normal, or a sign of some process left unfinished?

[rcc56]

Most likely, glue was used to install the frets, but wasn't cleaned up very well before the final sanding and polishing. It's probably a combination of glue, ebony dust, and steel wool particles.

[Marty Jacobson]

..Or buffing compound? I'd be surprised if this basic mandolin had glued-in frets.
Mineral spirits or naptha (lighter fluid) will wipe it right off if it is buffing compound, with no risk of harm to the finish.