Your Hydrangea appear to be sunburned. With the sudden onset of the high temperatures we have had this week, it is not unusual for Hydrangea to suffer sunburn.
Yes, Hydrangea do like sun but they prefer some afternoon shade too. From the photos you sent it looks like they are in a spot where they do not get shade and also may get reflected heat from the block wall behind them. You may want to provide some shade or perhaps move them in the fall to a more moderate location in your yard.
Thank you for your question