Our lawn has suddenly developed black slimy spots growing below the grass, on the ground. I saw your article on black slime on grass, but this is not really on the grass but below it on the ground.
Black Oily Spots on Ground
There are several hundred kinds of slime mold; some of them are found on rotting trees and bark mulch as well as grass, so I'm guessing this is just one that is on the soil. You should be able to wash it away with a hose, or sweep it with a broom or rake. If it doesn't break up and dissipate easily, it may not be slime mold.