If it is the entire leaf is blackened towards the late summer and fall, it is caused by aphid secretion. Sooty Mold. Control is achieved by controlling the aphids.
Otherwise, there is a leaf spot disease known as Marsonnina leaf spot or aspen leaf spot. The best control is to pick up the leaves, which are the source of reinfection, in the fall and cut away trees and branches to increase air circulation through the aspen.
Please see PlantTalk script "Aspen trees."
