I just put on a new set. I had the same strings on my upright for about 30 years, never got black fingers. Well I figured it was time and slapped on the new strings and Bingo! Black fingers! It's a bit oily, not much, more like graphite powder.
Is it possible that some new strings have a light graphite coating, maybe to protect them?
This is the first day with these strings and I got major black fingers... I guess it'll get less as time goes on .
I'll check this post after a few days and give an update.
Probably manufacturing residue- lube from rollers, guides, etc, and maybe polishing residue. I usually wipe new strings down with a rag wetted (not soaked) with denatured alcohol or 91% rubbing alcohol before installing. Anything further in between the wraps is insignificant as far as getting out to the fingers, and may help lube the windings and keep the core from oxidizing.