Can you please send us some photos of the issue in question? Unfortunately we cannot determine the cause of this condition from the description alone. Close-up pictures of the black patterns as well as an overall picture of the tree it's under would help (where we can see the entire tree, if it's not too large; if it is large, then at least the trunk and its base may be enough).

Is the black residue merely coating the grass, or does the grass pull up easily and have a mushy texture?

Miri

We received your photos.  The dark areas look like a type of root decay mushroom.  We suggest that you contact a certified arborist for an onsite diagnosis of the tree and evaluated for wood rot.   They can offer information on structural integrity, hazard information, and  the best way to proceed. https://www.treesaregood.org/findanarborist/arboristsearch 

Here is our page on wood rot of trees and How to Decide When to Remove a Tree

https://extension.umd.edu/hgic/topics/wood-rots-and-decay-trees-and-shrubs

https://extension.umd.edu/hgic/topics/how-do-you-decide-when-remove-tree

Marian

You are welcome.

Marian

There is no practical or permanent way to eliminate mushrooms. If they are a bother, simply pick and remove them as they appear. You can knock them over with a shovel and put them in the trash. https://extension.umd.edu/hgic/topics/mushrooms-lawns

Marian