I see this every year in spring on some dahlia buds. Although folks have wondered about insects being the culprit, the best most logical guess I've heard is that it is a bacterial infection, connected to rain/watering. I think this because normally I see it in the morning after dew, where I KNOW I didn't see it the night before.

The good news... it seems to be self-contained and doesn't spread, so you can literally roll the damaged buds off, and the new buds should be just fine. I wouldn't leave them on, as the plant would be putting valuable resources into a bloom that is already damaged. Let the plant focus on a future bloom you will really enjoy, instead.

I'm getting the same black/brown spots on the buds but not as much as your picture. I'll try to take a picture when I get home. When the flowers bloom, the tips of the petals are brown and the flower looks frazzled. I've been searching all over and found a blog that mentioned it could be from the hot temperature during the development of the buds and blooming of the flowers. We've had high 80's-90's a few weeks ago. I do hope that is the reason. All the leaves on my cactus dahlia looks good other than pests taking bites here and there. The blog also mentioned to deadhead the flowers so that new buds will bloom. I did that this morning and hope the new flowers look better.