Your tree is a tulip poplar. Several insects can live in tulip poplar trees. Two common ones are aphids and scale insects, both of which can secrete a sugary substance known as honeydew. Honeydew is clear and falls from the tree. A fungus, known as sooty mold because it is black, likes to live on this honeydew. The sooty mold is what you are seeing darken your patio and furniture.
You can have your trees sprayed for these insects, but it would be expensive and impact your beneficial native insect population. You can also scrub your furniture (bleach may help on the mold) and then periodically clean them to prevent buildup. Honeydew is water soluable, but the sooty mold is harder to clean. Many people avoid placing cars or other objects under tulip poplars (or anything white).
ECN--Thanks for the info. Will this black mold continue to rain down on my house throughout the growing season or is it just a spring thing?
If you are seeing more than normal this year it is most likely weather related. The rain suppressed predator and parasite activity with aphid populations exploding. It should get better.