Diagnosis
The blade bearing seal has failed. The lubricant (food-grade grease) is leaking and mixing with carbon dust from the motor wear.
How to Fix It
This is a contamination issue. The blade assembly is defective. You must stop using it immediately. Discard any food made with it. The blade unit is a sealed bearing assembly. You cannot clean or repair this. Replace the entire blade assembly with a new, official part.
If That Didn't Work
If new blades also produce specks, the issue is the motor coupler wearing down. Black dust from the plastic coupler can travel up into the cup. Inspect the plastic drive coupler in the base for excessive wear.
Heads Up
This is a critical failure. Ingesting bearing grease or plastic particles is a health hazard. Do not ignore black specks.