Ok, before you can even answer the other questions, I have some.
Would you describe the droppings of these birds as greenish and runny? Or is it more sulfurous and yellow in color?
I would assume all birds are rumple-feathered, correct?
Some diseases that have blackening of the comb as a major symptom are Erysipela, blackhead disease, and cholera (pasteurella). Darkening of the comb, as mentioned before, can also be caused by cyanosis (lack of oxygen) which could be caused by a number of things.
Honestly if I lost a bird to this, I'd have another bird tested immediately by your local vet.
Erysipelas can be treated with penicilins and also responds to tetracyclines.
Pasteurella (cholera) also responds to tetracyclines as well as sulphonamides (sulmet), and erythromycin and penicillin.
Blackhead is caused by a protazoa, probably the last of my choices of those diseases, and the medication for it is only available via vet.
There may be other illnesses, and again it could be secondary to cyanosis caused by something else like poisoning, respiratory disfunction due to a respiratory illness, etc. I'm not a vet, and non of us can truly diagnose these birds without a bacterial culture. Period. But we'll do our best.
Based on what I'm reading, were they my birds, I'd treat them immediately with terramycin. I'd give them full supportive nutrition. BUT BUT BUT you can't use any dairy products with cyclines. That means no yogurt. But since you're giving an antibiotic, and these disease already cause diarrhea, then you simply must give a probiotic. I recommend during and after treatment for your best chances. The easiest obtainable option is either a probiotic from the feedstore when you go there to get the terramycin. Probios is perfect - paste or powder will work, fastrak is good, just make sure the label says CFU somewhere on it for colony forming units and not just by-products or fermentation products.
Or you can get a thing of acidophilus tablets or capsules from the grocery/pharmacy/health food store. They're in the vitamin section. Give daily during medication at the opposite time of the day from when you put fresh medicated water out. Then give daily for a week thereafter and every other day for the following week.
On the antibiotics, the reason I choose the one I did (even though I normally don't like it) is because it specifically will treat the two major diseases that are listed for blackened combs. Sulmet only treats one. You'll need to always keep terramycin out of the light, and again - do NOT mix anything in the water other than the meds, and do NOT use any milk or dairy products like yogurt. They make the meds not work.
I'd get right on this. Feed them all boiled and mashed eggs yolks to get them eating. Give them crumbles. Talk to us later about how to give the probiotics. Test if you can, if you've lost birds. If you lose a bird and you want to test, cool the bird in running water. (Don't wash off any signs from the outside like congestive drainage etc.) Just cool the bird, pat dry with paper towels, wrap in a layer or two of paper towels, secure within a plastic bag, and refrigerate. Do NOT freeze. THat distorts tissues and organs. Bring to your vet for at least a necropsy within 24 hours.
Keep the birds completely separate from all of your other flock. handle them last, changing clothes and shoes in between. wash your clothes, save one pair of shoes for only the sick bird room. Wear gloves.
Decontaminate all the feeders and waterers. healthy birds' first, then sick.
