Oh yes, lots of bugs and rain!
What does the poop look like? (photos)
Blackhead is usually seen in Turkeys, chickens can get it but it's not that common.
My rooster does seem to get those black spots (insect bites) more than the hens. Here's what happens to mine. He lets the hens pick at those spots, they bleed, get a few more scabs, they pick some more and on and on it goes. I've about given up trying to heal his comb. I've had him separated for a few days for another reason and his comb is looking better. Watch your fellas to see if the hens pick at their combs.
Fowl Pox is another possibility. Since I see no oozing, it's most likely dry form. It's transmitted by mosquitoes. Seems a large rooster comb is targeted more so than a hen.
Fowl Pox will usually resolve on it's own, but you can apply some Iodine to help dry the scabs - don't pick the scabs off, just let them fall off.
Keeping the water clean is important if it's Fowl Pox, it can spread.
Diarrhea can be caused by a number of things - drinking more in warm weather, eating "treats" like watermelon, cucumber, etc., worms and bacterial infection are some of the most common. Having a fecal float to rule out worms is usually a good idea.
Here's more info on Fowl Pox:
http://www.the-chicken-chick.com/2015/07/fowl-pox-prevention-treatment.html