It sounds like your cherry tree is afflicted with either thrips or aphids. Regardless - the treatment is the same - insectical soaps or neem oil.
For more information on thrips and aphids and a course of treatment, please visit the following links:
my cherry trees have curling leaves with tiny black insects in the curl
my peach and plum trees just have curling leaves with out any bugs
What spray do I use to treat my trees and get rid of the leaf curl and the tiny black insects?