It's very common in this area - hard to find a perfectly clean lilac around here. But the good news is the blight is more cosmetic AFA the lilac is concerned than causing serious damage. But this same disease affects a great many different plants so you need to be on alert for signs it may have infected other susceptible species. Good garden sanitation - raking up and disposing of leaves and debris, etc. - will help to keep things in check.
Pseudomonas syringae in the PNW

Thanks, gardengal. I remember some deformed leaves before but this year the flower buds were affected. I've also been pruning (beyond deadheading), so maybe that's allowing the bacteria to enter. I just reshaped it, so I'll leave it alone now.