How do you treat blood blisters?

Protect it. An unpopped blister represents a closed sterile environment. Yes on occasion one can become infected then it is time to open it, however the risk of infection in a dressed open wound is still higher than that of an intact blister. The blister will gradually resolve with time.
Depends on size. Small blisters (1 inch) will normally heal on their own. Do not let anythng rub on it. You can use a doughnut shaped pad to pressure of it.Keep it clean and dry. If it breaks do not remove the skin flap unless it is dirty. You can apply an anti-biotic ointment and loose bandage.Change it daily and do not wear shoes that will rub on the area. See dr. For increased redness, drainage, fever.
Blisters. It does depend on the location and size and cause and pain or not, but usually benign neglect.