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It could be either black spot or another fungus. This article will help with black spot: (it referes to black spot on roses, but it applies to all plants) https://www.gardeningknowhow.com/ornamental/flowers/roses/black-spot-roses.htm

If you don't think black spot is the problem, treat the plant with a fungicide.

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
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

It is possible that there are air pockets in the soil that are affecting the roots, making it difficult for the plant to take up water or nutrients. I would step firmly on the root balls to help collapse any air pockets. Flood the area with water to help fill them in.

It may also have a fungus disease, so I would treat the plant with a fungicide.

If no signs of life seem to appear, then go ahead and replace the shrub.You can grow the shrub in a pot. Just be sure the container is large enough to accommodate the plant and provides adequate drainage. It can also transplant fairly easy, but make sure it is done while dormant and get as much of the root system as possible. This article should also help: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm

This article may help: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm

The only thing I can think of would be the pH level of your soil, which affects the color in hydrangeas, so it isn't really unusual to see various shades of color depending on the soil pH. This article has information on changing hydrangea color: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/change-hydrangea-color.htm