DIAGNOSIS: That's wear material from the jar's plastic drive coupling rubbing against the metal motor shaft socket. It's a mix of rubber and ground-down plastic.
FIX: Unplug the unit. Use a cotton swab dipped in isopropyl alcohol (91%) or a mild citrus-based cleaner. Scrub the black residue from the rubberized ring inside the base's socket. For the metal teeth, a soft toothbrush with baking soda paste works. Wipe clean with a damp cloth and dry thoroughly.
IF THAT DIDN'T WORK: If the residue is ingrained, the rubber ring itself may be degrading. You can sometimes peel it out and clean under it, but reassembly must be precise. Consider replacing the entire base if cleaning doesn't stop the shedding.
HEADS UP: This residue is normal over time but excessive amounts mean the jar isn't seating perfectly, causing extra friction. Ensure the jar is twisted on firmly. The residue is not toxic but is messy.