Used coffee grounds are put in the garden but with a pH around 6.5 they won’t affect color.

tj

As noted, coffee grounds are almost neutral afa pH is concerned. You can apply as freely as you wish but they are not going to alter pH anywhere close to changing flower color.

To be specific, the flower color of bigleaf hydrangeas is determine by the availability of aluminum in the soil - an acidic soil just provides greater availability for the plant being able to take up this element. However, you can have an adequately acidic soil but with low Al levels and the blue flowers will still not be produced. The best way around this is to use aluminum sulfate to achieve blue flowers, as it is both an acidifier as well as supplying the necessary levels of Al.

You should also be aware that caffeine has a negative impact on plants, altering cell and root development and resulting in stunting....or if applied heavily enough, plant death. Fresh, unused grounds contain about twice as much caffeine as used grounds so avoid using these in the garden.

How about used decaf grounds?

Perfectly OK to use in the garden but not gonna change the flower color on a hydrangea!!