Could you provide some additional photos? From further away? It doesn't look like something you could treat with a product. Is it hard or soft? Can you peel it off? It could be some wood decay fungus. Have you renewal pruned your lilac lately? That is, Removed some of the thick older stems? Pruning in the fall will reduce spring blooms but if you are removing the older stems all together the younger ones will still flower.

https://lancaster.unl.edu/pruning-lilacs

Emily