What kind of hydrangea is it? Are you growing it in a pot or outside? Can you post a picture of how the shrub looks and a few more of the leaf damage? Yur symptoms could be one or two separate problems so pictures may help.

Leaves turning black can be a serious fungal infection like Anthracnose. Leaves curling could be a few things, starting with drought stress, chemical damage (from insecticides or fungicides applied nearby), the hydrangea leaf curl insect, aphids or salt-stress (salt stress applies more to potted plants). It could also be leaftier larvae. But most of these would not cause the leaves to turn black unless -like aphids- the insect(s) secretes something like honeydew, which produces a black sooty mold.

Investigate if you have a insect infestation by checking all the leaves (top and bottom) as well as the stems as best as you can. Do this during the day and again at night since some pests only show up during the night. You can also look under nearby rocks or move the mulch around to see "what moves".

If you determine that you have a pest infestation, you need to identify so you can apply appropriate controls for it. YOu can take samples to a plant nursery, look in boks or send the samples to your Agriculture Extension Service. Once you know what ails the plant, you can apply appropriate controls that work against that pest.

For example, aphids can be controlled with a strong jet of water (they are "brittle" insects), with a soap/oil spray mixture or you can release ladybugs. You can also make your own spray: mix 1 tsp. insecticidal soap with 1/2 tsp. horticultural oil in a 1 qt water spray bottle. Or just go buy horticultural oil spray at a plant nursery.

Luis

It is an Annabelle and is growing outside in the ground. When you mentioned chemical damage it rang a bell. My husband used something on the grass to get rid of Creeping Charlie, it could have seeped into the root system, I suppose. Do you think It will kill the plant or might it recover another year? Sorry I do not know how to post pictures on this site. Thank you for your input.