They are very good as a weak fertilizer but there is not much acidity left after making coffee, meaning not the high amt that you would need to lower the pH significantly.

Because they are still good for your plants, I would add them throughout the growing season but stop all fertilizers somewhere around July-August so the fertilizers do not make the plants "stay awake" in the Fall. Only fertilizers rich in nitrogen interfere with dormancy (although applying too much of a weak fertilizer could eventually do that too).

I even have one Border Collie that loves the smell of coffee grounds so much that she will cover herself in coffee grounds! She loves it when I bring some from Starbucks. Hee hee hee! Dogs will be dogs. It is interesting when they return back to the house, the BC smelling like coffee and the others smelling like mint.... I can tell where they have been!

According to my resources (and not personal experience), acidity is necessary but not sufficient for color change to blue (you have to have available aluminum as well); acidity makes aluminum available to the hydrangea. Also, you cannot change the intensity of a color with acidity or aluminum.

The coffee grounds will add some nitrogen to the soil, which may help the color a bit (see link).

Here is a link that might be useful: Color Change