The leaves on our tree outside gets black all over them and everything under it including the deck turns black what is wrong with the tree?

Not knowing the type of tree or your area of the country, it's hard to be specific. It sounds like a type of mildew that grows prolifically in the presence of aphids and thrives on the "honeydew" (the waste excretions) of large infestations. Imidicloprid (a systemic insecticide) should help, but the tree should be diagnosed in person first. Always follow the label instructions.