Linda.  

 I don’t suspect that over watering is the issue. Some leaves look ok, others look ok but are showing a few dark specks and others don’t look ok and have a lot of dark spots and areas. Looks more like a disease to me

  Lilacs get a disease called Powdery Mildew. With this disease the leaves have a dry grayish white spots and sometimes the leaves look like they been dusted with flour. Have you noticed anything like this on your lilacs? I don’t see anything that looks like this in the photos.

  Another disease lilacs get is Bacterial Bight. The symptoms of this disease better match with what’s seen in the photos. This disease starts out as small dark spots then the spots coalesce to form dark areas as the disease progresses.

  As the name says Bacterial Blight is a bacterial disease. 

  To control this disease one option is to plant lilac types (cultivars) that are resistant to this disease. Sanitation combined with other good cultural practicies can provide control of lilac Bacterial Blight.

  Michigan State University recommends these cultural and sanitization practices to control this disease. 

  Blighted shoots should be pruned well into "green" cambial tissue, make pruning cuts at least six to eight inches below any visible signs of infection. This will also stimulate lateral bud development for new shoots. Never prune when leaves are wet or during wet weather. Pruners should be cleaned with a disinfectant between cuts. (Regular rubbing alcohol (70%0 or a 10% chlorine bleach solution can be used to clean tools.)  The tissue that is pruned out or crumbles as leaf litter is still infectious and should be removed and destroyed to prevent further disease spread. If practical, eliminate overhead irrigation, which spreads millions of the bacterial cells (literally) to healthy tissue. Good plant spacing will help reduce short distance spread and promote more rapid drying of the foliage.

  The link has more information this disease, lists some resistant cultivars and controls. The link mentions some chemical controls for this disease. But these chemical controls need the right timing and chemicals to be effective which might them hard for a homeowner to use. Contacting a professional might be a good option if you are considering using chemicals.

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 Purdue