Hydrangea is susceptible to leaf spot diseases especially if grown where air circulation is poor and when watered overhead. The shrub should survive if it can store enough energy in its roots to overwinter. Cool weather will turn the leaves purplish as the plant starts to go dormant for winter.
Right now keep the soil moist until the ground freezes. Water from below the leaves, keeping leaves dry. Rake up any fallen leaves and throw them away to reduce any fungal spores that might overwinter. Mulch around the root zone 2-3 inches deep with fresh mulch, but don't push mulch up against the stems. You can spray leaves with a garden fungicide, following label instructions, to stop any fungal growth where it is now- it won't cure the damage, just slow or stop it.
Chlorothalonil is one common fungicide available under several common retail names you can find in your garden center.
Here is a link for your reference-
Next spring watch for the first signs of leaf spots, especially during rainy periods, and treat leaves with fungicide to preventatively. Thank you for using our service.