I am not a peony expert, but here is a link that 'may' help you a little. How long have you had the plant? Have the number of blooms decreased over the years? Is it still fairly young?? Did you recently transplant it? Stuff like that may help one of the experts tell you whats going on with it.

Here is a link that might be useful: peony diseases

Also, have you applied any fertilizers, lime, or anything else to the peony? I once applied a chemical fertilizer to my peonies in the spring hoping it would help them grow. All it did was turn the leaves yellow. I never did that again!

I have 5 tree peonys. Only one of them, the leaves has turned from green to a red or maroon color. Another one had done the same thing, but now the maroon color is fading and you can see the green again, it's almost like it is in transition. All the other plants area a beautiful green color.