Answer: Hydrangeas need an organic, evenly moist yet well drained soil. The soil should be kept damp like a wrung out sponge, not sopping wet/saturated and not dried out. A new plant needs special care to watering since it is not yet fully rooted. Sometimes the root ball with the original soil will dry out faster than the surrounding native or garden soil, so you need to check both with your finger to see if you need to water. Sometimes too the plant fails to root into the surrounding soil so the roots are constricted, such as having been planted into a heavy clay soil or with encircling roots, this can cause wilting.
Based on your description, I am not certain what is happening to your plant. If it is wilted in the morning and the soil is dry then it definitely needs more water. Wilting in the late afternoon can be due to heat stress rather than moisture stress; if the soil is still damp you would not want to water it. If it is heat stressed, the plant should be perky again by morning.
Overwatering can cause wilting. Root damage (due to overwatering or poorly drained soil or root disease or even planting too deeply) can also cause wilting as a symptom.
The blackened leaves are worrisome, and may indicate a disease problem. I would suggest you consult with your local county extension to obtain a specific diagnosis of the wilting and discolored foliage. If a chemical control is needed, they will have the most up to date information on what to use and how/when to use it.
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