These are classic symptoms of a common disease called Elsinoe Leaf Spot; we see it fairly routinely in wet years on dogwood petals (and sometimes foliage as well). We've had the weather for it, at least during periods of damp, cool conditions this spring. Plant infections in general can be highly weather-dependent, being more or less severe any given year based on temperature and rainfall trends.

There is no curative treatment for this disease, but fortunately it tends to be more of a temporary cosmetic issue than anything overtly threatening to long-term tree health. It wouldn't hurt to rake-up and dispose of the fallen bracts (flower petals) once they are spent, to remove some of the spores from the area, though it's not necessary and it won't be a way to completely prevent the chances of reinfection next year.

Miri

You're welcome!