Thanks for the question.

You have recently had some very high temperatures with drying winds. Many plants, including lilacs, will experience dehydration in the leaves (this is why they are curling) and leaf discoloration. In the vast majority of instances no harm is done to the plant and it will recover. In the meantime, be sure to water on a regular and frequent basis. Do your watering in the early morning if possible. If you wish you could remove at this time the blackened and discolored leaves. The following will describe this sun scorch in further detail:

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/environmental/scorch.aspx

Good Luck