Black Spot on Ground Beef?
Obviously hard to know for sure. The laws and guidelines dictate use of food grade equipment so if a piece falls off and ingested, it does little harm.  If you cooked it, then you have even less to worry.

As a precaution, I suggest taking it to the store and asking about it so other customers don't trip on the same issue. Harmless or otherwise. If they don't know for sure, then suggest to them to send in for a lab test. 

In descending likelihood, it could be:


a piece of buna rubber broken off from a gasket (grinding machine, or other). Not a hazard. It's not far from chewing gum ;)
a piece of meat that came in contact with the lubricant oil in the machine. Also usually food grade. They turn dark after a while. 
sometimes the meat branding ink on the outside ends up in the grind (also food grade). Though they are usually dark blue. 
piece of old blood clot, liver or other organs. 
not likely here, but I have heard Listeria shows up black. Though i'd imagine it would spread and look more fuzzy/furry. Most bacterium die during proper cooking.