Hi Amit and thanks for your amazing pictures of the black cherry aphids covering your cherry and plum trees. These little critters are usually controlled by ladybugs and other predatory insects, but you have such a severe invasion I think you will want to try something right away.

You should first try a really strong stream of water to knock them off. This is harder now that the leaves have started curling but still worth doing. This big of an infestation can really damage your trees, so get as many off as you can by water or hand wiping if the trees are small enough. If you can get the population down to a manageable level, the predators may be able to take care of the rest. Lady beetles, syrphid fly larvae, and green lacewings will all enjoy a feast of aphids.

Try not to over water the trees. If you fertilized this spring with nitrogen, you may have used too much, which makes the tree more susceptible to the aphids. 

The only chemicals available are highly toxic to bees, so I won't recommend using any of them at this stage.

Good luck getting rid of this pest!