I still say meat spot. It was probably brown in the raw egg (which of course you couldn't see because you hard boiled the egg) and it turned black with the cooking.
Meat spots are different from blood spots, but both are harmless.
Meat spots are tiny bits of tissue from the reproductive tract:
http://www.thepoultrysite.com/publications/1/egg-quality-handbook/29/meat-spots
While blood spots are just that, tiny specks of blood:
http://www.thepoultrysite.com/publications/1/egg-quality-handbook/28/blood-spots
Meat spots are different from blood spots, but both are harmless.
Meat spots are tiny bits of tissue from the reproductive tract:
http://www.thepoultrysite.com/publications/1/egg-quality-handbook/29/meat-spots
While blood spots are just that, tiny specks of blood:
http://www.thepoultrysite.com/publications/1/egg-quality-handbook/28/blood-spots