what is happening to my hydrangeas. There are brown blotches on the leaves?

It could be some kind of fungal leaf spot, or just damage from creatures eating the leaves - check the stems and behind the leaves to see if you can see any kind of infestation. Be careful what you use to treat it with though - hydrangeas can be very sensitive to treatment sprays, so never spray in bright sunlight and always make sure the plant has been well watered first.