Answer: Since it is setting buds and growing well I do not think it is sun-related. (In too little sun the number of buds is reduced.)
It might be a case of a disease such as botrytis or possibly anthracnose affecting the blooms. I would suggest you work with your local county extension to see if they can give you a specific diagnosis and if a chemical control is needed, they will be able to tell you what to use and when/how is best to apply it. They may appreciate photos of the blooms and also the overall plant, or possibly fresh cut samples of slightly and badly affected blooms in a closed plastic bag (keep them cool so they don't wilt).
In the meantime, be sure to do a thorough clean up of last year's hydrangea foliage and remove any hydrangea prunings from the garden to limit sources of reinfection. Also avoid watering in the evening or wetting the foliage/buds/blooms when you water and be sure you are not overfertilizing as this can contribute to certain disease problems. I hope you have better success this summer.
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