Ok here's everything I've found on either darkening of the combs or blue coloration of the combs.
If it's a bacterial disease (and there's really no way of telling for sure at this point without testing, which I would highly consider in case of loss of any more birds - any more illnesses), the diseases all seem to be sensitive to terramycin, aureomycin, and penicillin. My choice above the others would be Aureomycin. Then Terramycin. Perhaps Penicillin injectable if you are capable of giving shots. (It's really quite easy and Penicillin G procaine is readily available). I'd do this immediately just based on symptoms.
If it's viral, there's no treatment.
We haven't ruled out excessive intake of salts. Given the current weather conditions, and the fact that this is not yet ruled out, if you're giving electrolytes I'd stop immediately. Instead I'd give a non-salt version like ACV until you medicate, at which time you cannot. Darkening of the comb is a symptom of salt poisoning. Also eliminate any water softening products or well issues that might cause this.
If you feed your birds anywhere in the vicinity of a salt lick, or if there are salt licks near a source of water, I'd fix that. Otherwise sometimes dehydration can cause it. But generally if non-viral and non-bacterial, salt poisoning is to be ruled out.
Based on this, I'd still say to do this:
Treat immediately - all birds sick or not (and mark the day because I rarely EVER say this) with Aureomycin or terramycin. Remove all electrolytes from water immediately. While medicating, start treatment with probiotics which should NOT include yogurt. With hold all sources of dairy until at least a day after treatment. Use acidophilus tablets/caps from the store, or a prepared livestock probiotic.
Feed all birds their standard food, decreasing grains in case of crop stasis d.t. antibiotics. Monitor food intake and water intake very carefully.
After the full antibiotic course, treat for two weeks every other day with the probiotics. If any other birds are lost, take to the vet for testing. One of the diseases not ruled out is AI. So this is something not to be messed with considering the rate of mortality already shown. That's exactly why I'm recommending antibiotics - to rule it out at home.
That's my recommendation.