There is only 2 ways. Walk in a swamp and get leaches that attach to you. (They will equip to your equip GUI. Wait for them to get fat then take them off.) It will make the black one decrease. (And make the orange go up.(Tip the orange is your perma HP you WANT it higher.(please note i don't actually mean perma.)))
The second way which gets up around 20-50 I forget. Its a Gauze. You can shear a sheep for it.
Gauze:
http://havenandhearth.wikia.com/wiki/Gauze (please browse on it for a little while. And check out the requirements and everything.)