I could not find the caterpillar in the picture, but the dark brown dots do look like caterpillar droppings (frass).  It is possible that the leaf problem is caused by a leaf miner. This insect begins as a leaf miner in early June; later the caterpillars emerge, spin webs around the curled leaves, and further skeletonize the leaves. The foliage is badly disfigured by this process. There are two or three generations a year, the last one in September. Remove the affected leaves and clean area beneath the shrubs to prevent reinfection. Sprays with malathion , a nicotine spray or neem may help, but once the damage is noticed it is usually too late to take action. The insecticide must be applied before the leaves are rolled.

Another possibility is not an insect, but a disease called Bacterial blight. The most visible symptoms of bacterial blight on lilac are the blackening of new green shoots, leaves, and flower buds during rainy, mild spring weather. Early symptoms are visible as dark lesions on new shoots that quickly girdle and cause wilting of stems, leaves or flower clusters. Leaf symptoms are irregular dark spots on leaves sometimes ringed by yellow halos. Leaf spots quickly grow together and blight entire shoots. Stem lesions on green twigs start as dark streaks that quickly girdle and cause wilting and blackening of shoots.

Prune or thin lilacs in early summer to promote better air circulation and allow time for wounds to heal before next season. However, quickly prune out diseased shoots during dry periods, and remove the clippings from the area.

Sites to visit for more information:

http://www.herbs2000.com/flowers/li_pest_dise.htm

http://plantdiagnostics.umd.edu/level3.cfm?causeID=52

To me it does appear to be most likely the lilac leaf miner, but I wanted you to be aware of the bacterial blight problem, too.

Hope this was helpful. Feel free to contact us again if you have further questions.