Blisters and sores are two common symptoms of Fowl Pox.
Fowl Pox is a disease caused by a virus (its not related to Chicken Pox in humans). Fowl Pox does not have a treatment, and runs its course in about three weeks. It isn't usually deadly in the dry form, which your bird appears to have. However, it can be deadly if it progresses to the wet form, which causes lesions and scabs in the throat as well.
The main carriers of Fowl Pox are mosiquitoes, flies, and other flying insects. To reduce the spread of the disease, treat your chicken area for flying insects. This will eliminate most of the carriers.
Keep infected birds as stress-free as possible to prevent a secondary bacterial infection. Give electrolytes and probiotics, and make sure that they eat and drink. You might want to put your birds on a course of antibiotics. Oxytetracycline like Duramycin, Terramycin, and Tetroxy HCA-280 is a good antibiotic, as it is broad-spectrum. Make sure that you don't give probiotics, yogurt, apple cider vinegar, or other dairy products while using antibiotics, as they will interfere with the success of the antibiotics.
To make your birds more comfortable, mix together some vaseline with sulphur (I believe that sulphur can be found at garden stores, but I'm not sure). Put this mixture on the scabs, and they will soften so that you can remove them. You might not be able to remove all of the scabs, though.