Lilacs are hardy enough that cold shouldn't have been an issue in the metro Boston area. Have you looked carefully at the backs of the leaves to see if there are spider mites or aphids? Both cause leaf issues. The flower buds were set last summer, so think about conditions then. Were these new plants? Was the weather unusually dry? What is your soil like? Is it near an area that gets winter salt? If the plant is stressed due to whatever is causing the leaf curl, it may not have had the energy to set flower buds.

Lilacs need 6 hours of sun to bloom well. They also like neutral soil. I will often put a handful of lime in the backfill when I plant them Mostly our soil in NE is acid. Mine is 5.0.

The other thing to look for is borers. They will bore into the oldest branches of a lilac and damage it. Look for holes and/or frass in the largest branches.

You can always rejuvenate an older plant by cutting out one third of the oldest branches about one foot from the ground. If you continue to do this for 3 years you can completely renew the plant. Be forewarned that this will make it smaller than before. But it will grow back to full mature size.

Steve

Thanks for the suggestions! I'll inspect them for pests tomorrow. They could probably stand to get a bit more sun than they do, but they've always flowered quite reliably before, so I don't think lack of sun is the only reason. I did rejuvenation prune one of them three years ago (there are three plants total and I pruned the leggiest one) and it has come back fine--it had a couple of flowers last year. But none this year.

My inspection showed no signs of aphids, mites, or borers. The leaves don't really look that bad--primary symptom is lack of flowers. Last year's conditions weren't remarkable, it doesn't get salt, and very close neighbors have plenty of blooms on their lilacs. It could be a sun issue, but it seems rather dramatic a decline in a single year. Still not sure what's going on!

Pictures might help