Ok, so you know when your hallow right, and you put your sign on the ground, and someone summons you. You join their game, and you take some damage from fighting enemies. And you want to heal, well your flask is greyed out, and you can't use it. Well I learned something else to help with that. If you have the miricle spell HEAL, you can use that to heal yourself and you can use that five times.
Last night I was in the depths and since I didn't know the area, I wanted to join someones games and go through the area and learn it, and know that if I die, I won't loss my souls or humanity in someone elses game, so this allows me to explore with no risk plus have the help of another person. So someone summoned me(FINALLY) and I took some damage a bit, and I noticed when I was scrolling through my spells, my miracle spells wasnt greyed out like I thought it was, so I used it just to see if it would work, and yup, I got healed.
So my advice to you guys, if you havent gotten any miracle spells nor the healing spell, and you play co-op a lot by joining people games, i'd advise you to get that spell from the fat guy in firelink shrine. You need at least 12 faith to use the spell and the talisman.