Well Turtle mom your going to have to "investigate" and see what it could be. Not sure you have the same bugs down there, but the caterpillers we have here start showing up in the late spring, and they leave little tiny pellets everywhere. They don't last all summer cause they change into some moth later in the summer. They can come in very large numbers and eat lots of the freshest green leaves, make nests of cobwebs in the trees where they sleep all together, by the thousands! People here hate them and spray chemicals to kill off everything in sight, including there selves! I am lucky nobody sprays around my house, as I hate chemicals and happen to like the caterpillers. I believe the trick to gardening and ponding, is to create a very natural balance. It took years to properly balance my pond with all the different types of bugs, everything from water Striders to dragonflys and even leeches! Did you know horse leeches suck from both ends and don't drink blood but live off tinys "buggy" things in the plant! Opps off topic, that's another story.....
Yes, back to the black blotches you are finding on the ground... my first thought is to see if they are located around trees mostly, then you can examine the trees for further clues. Look to see if anything eating the trees, or perhaps some other critter may be involved.
You will have to examine the area and reportany new clues cause now I am wondering.... humm