If the tree is not too big, spray with insectcide.....but not if you want to eat any cherries. The birds will get them if you don't. If the blackfly is on new growths....prune them off. The tree won't mind ,as it's being got at anyway.

good morning from bulgaria, where we have planted our own small fruit orchard, which isoccasionally attacked by ayphids, blackfly etc, we use a carbolic base spray, we can buy small philes of carbolic here which we add to 4 litres of water. Maybe your local chemist or vet even can help, if not carbolic soap left in a bucket of water and sprayed over trees. We had terrible blackfly on plum trees this year it worked a treat. The reason we use carbolic it doesnt hurt the other insects butterflies etc as we are trying to create a garden for friendly bugs! good luck enjoy your cherries maria