Certified GKH Gardening Expert
Yes, they like the acidity that coffee grounds can add to the soil. A cup or two (depending on teh size of the plant) once every few months is fine.
If you are having issues with blooming, you may be lacking phosphorus. If there is a lack of phosphorus in the soil, plants will have little to no blooms. You can either use a phosphorus rich fertilizer or add some bone meal to the soil. These articles may help as well: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm