Answer: Based on your description, I suspect it may be botrytis which is a fungal infection. If so, the dry looking blooms are likely to develop a fuzzy gray mold on them. Remove the affected flowers and dispose of them in the trash, then make sure not to wet the foliage or blooms when you water the plants. Make sure your plants are spaced far enough apart for good air circulation. Do not work on the plants when they are wet from dew or rain.
If you water (and there has been quite a long dry spell in your area) you need to water slowly and deeply so it soaks down to where the roots are. Dahlias need an evenly moist soil, it should be wet like a sponge but not saturated/sopping wet. When in bloom, they need about two inches of water a week as a rule of thumb which is more than they needed, maybe double what they needed before they started to bloom.
You might also check with your county extension to see if they are aware of any particular problems with dahlias in your area in recent weeks. I hope this helps.
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