Black spots on my Lilac Bush
The black spots are a bacterial leaf spot that occurred this spring. It is highly weather-related. There is no treatment now, though you can clean up and dispose of fallen infected leaves to reduce the amount of infectious material in the area.
Your lilac is looking very thin and sparse. After 40 years, the location may have become too shady for it. (We see a mulberry tree in the third photo.) Perhaps you can prune other plants to afford the lilac more sun. It should be producing many new shoots from the base each year.
Lilacs like alkaline soil. You may need to raise the pH with lime. You can do a soil test to determine how much lime may need to be applied. Here's info on soil testing, including a video on how to take the soil sample and where to mail it (we like Delaware University): https://extension.umd.edu/hgic/topics/soil-testing
Next spring, you could try renovation pruning. In March, as soon as the leaf buds are about to open, prune down to 8-10 inches. Fertilize. Water during drought for at least 2 years.
Here's more: https://hortnews.extension.iastate.edu/1993/2-10-1993/lilac.html