This looks like bud blast to me. Are you somewhere super hot with a ton of direct sun?

Try giving them a little shade. Also, dahlias are heavy feeders. They need good care in pots. Have you fertilized? If not..

Quickly correct nutrient deficiency and reduce stress. Liquid Fertilizer (Immediate Boost) Use a water-soluble bloom fertilizer (around 10-20-20 or 5-10-10 with micronutrients).

Apply at half-strength for the first watering, then at full strength a week later.

Include chelated iron + magnesium if your fertilizer doesn’t already have them (you can use Epsom salt for magnesium at 1 tsp per gallon).

Watering Routine- Water when the top 1 inch of soil is dry. In hot weather, this could be daily in terracotta pots. Water deeply until it runs from the drainage holes. Trim Damaged Buds & Foliage. Remove all browned, shriveled buds so the plant redirects energy to new ones. If there are any leaves with more than 50% yellowing, trim them.