it looks like an abscess of sort (barring any other such boils or blisters on the comb or feet which would indicate pox)... abscess in a bird must be manually removed as it will not be "reabsorbed" into the system such as is the case with mammals... you can coat it with iodine (same thing as what you do with pox lesions)... however this may not (and probably will not) "cure" the lesion and it may even get worse. I would suggest you go to a vet , have it lanced and if necessary he will give you an appropriate antibiotic ....