Any idea what this is? Suddenly appeared and looks like oil?
Black Oily Substance on Lawn
Certified GKH Gardening Expert
This is a common fungal infection. There are many ways to get rid of this from boiling water, to peroxide and water mixed half and half, to wettable sulfur.
I recommend leaving it, however, as it can actually be good for your lawn! Just unsightly. It will clear itself up over time, and even help combat other infections in the yard.
This article will give you more information: https://www.gardeningknowhow.com/lawn-care/lgen/black-substance-on-lawns.htm