I have looked at my neighbours grass and found it has black sooty deposits on it.
It grows in a tree enclosed garden with an NE aspect on clay soil.
I am unsure what it is as I have never seen it before.
I will try later to get a photo if this and post it up to see if any assistance can be offered.
I would guess that the trees have an insect infestation and the black deposits on the grass are waste excretions from these insects.
Or it may be a moss infestation that has been treated with weedkilller!
No lawn or weedkiller treatment since they like me do not use chemicals.
Must be from insects then, or is it something else!!! ???
A picture would help, but I suspect it might be Dog lichens. It's a weird, black thing that appears grass that's in poor condition: compacted, poorly drained and shaded. All the usual treatments for turf apply: rake it out, treat the whole lawn and improve the aeration, drainage and nutrition of the lawn.