What should i do if my boil is filled with dark blood but no pus?

How big is the boil and its location? Most boils break and drain by themselves. Yes, sometimes pus mixes with blood and the content looks like blood.

Since a boil is usually caused by a staph infection, it is filled with white blood cells that are part of pus formation. If your lesion didn't start as a tender,red swollen area & didn't contain any pus, it would be a hematoma ( blood collection) but not a boil. A boil can contain both pus and blood, however. An MD could examine and advise you as needed.

Boils usually heal or burst and heal in a few days. They may have what looks like pus or may not If it does not heal in a few days pl see your MD