If they are aphids, hose them off. Aphids can be green, brown or black. They are soft bodied, so if you support your petunias in one hand and use a nozzle set to flat (NOT JET !!) hose those beggars off. It doesn't matter if they go in the garden because the water pressure will have killed them. If there are Loads of them, you may have to resort to chemicals. If you hose them off, be dilligent, check back the following day and every few days after that to make sure that you got them all.