Question
We've lived here 7 yrs and this is the first time we've had the locust leaf drop. And it started in June and it's like Fall out there. The leaves appear sickly yellow with darkish spots when they fall. The leaf drop seems to be creating blackish leaves on our dahlias and one of our hydrangeas affecting the plants flowers.

Answer
Anthony:
Many things can cause an early leaf drop. Usually it is often related to some unfavorable soil condition or sometimes disease or insect.  If the weather has been unusually dry, this can contribute to yellowing and leaf drop.  Be sure it is getting ample deep waterings during the summer months.  Also, take a look at the trunk for evidences of loose/damaged bark. This may indicate some potential decay within.
As far as the black color that you are seeing, using a wet finger or paper towel, see if this black rubs off of the leaves.  If so, it is sooty mold, a fungus that grows on honeydew (insect poop).  Honeydew can drip down on lower plants from upper leaves. Its sugary and provides a good substrate for the fungus to grow.  Cure: manage the insects that causes the honeydew.  Look your plants over for insects feeding on the leaves and have them ID'd by your local garden center/nursery to find the best insecticide.

Regards

Steve