Hello Barb.  Thank you for using Ask Extension. 

According to the research, coffee grounds won't make your soil more acidic, which is something hydrangeas love, as you know.  Coffee grounds can have some benefits.   If tilled into the soil, they can improve the soil tilth.  They are a small source for fertilizer, and they do seem to repel some pests.  So, in general, the research says if used properly, you may get some minimal benefits.  

Here is an old article written from a University of Washington professor.  https://s3.wp.wsu.edu/uploads/sites/403/2015/03/coffee-grounds.pdf

Here is a more recent article from the Oregon State that describes ways to use coffee grounds. Take note that when the article mentions that coffee grounds are a good source of nitrogen, they mean in compost piles, not if used directly in the soil.  

 https://extension.oregonstate.edu/gardening/techniques/coffee-grounds-composting

I hope that  helps.