Answer: Without seeing the plant, it is difficult to make an accurate diagnosis, but since Spirea is in the rose family, it is quite possible that the black leaves are due to a disease called fire blight. Fire blight is common on pyracantha, apple trees and pear trees, but also attacks roses, spirea and other landscape plants. You can cut off the affected plant parts, disinfecting your pruners after each cut. If you remove the disease pathogen, you may be able to keep it from completely engulfing your spirea.
As for the lilac leaves, unfold them to see if you find little caterpillars inside. If so, these are called leaf-rollers (for obvious reasons) and you can control them with Bt (Bacillus thuringiensis), sold as Dipel in most nurseries.
Best wishes with your garden!
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