my friend grows bonsai trees and purpuse fully takes all the leaves off mid seasen to grow smaller leaves so theres always hope . there is always hope . what you need too do is keep an eye on the tree from when its budding in spring and you will catch any flies before they get to bad . I would take off any leaves that are obviously dead and bin them . don't compost them if you can .

Any chance of a photograph? And what did you spray with? Check backs of leaves, stems, branches and the trunk for signs of anything that shouldn't be there - scale insects, soggy bark, anything at all.

Just to be on the safe side I's collect all the fallen leaves and dispose of them (not in the compost as NP says)
Are you sure it was black fly? I've not come across that infesting a cherry before.

What kind of soil did you plant it in, how big a hole did you dig, have you kept it watered during its first season? And as Bamboo says, what did you spray it with? Not all garden sprays do the same job.