If the lawn treatment has a mosskiller, then it'll be the moss blackening and dying, as it should, and should then be raked out. Otherwise you're either over applying the treatment, or not watering it in within four days, either from rain or hosepipe, assuming the treatment is granular in form.

Bamboo

you hit the nail on the head its black lawns all around me apart from my house front lawn I told them to water but they ignored my advice I shant say again lol plus the xtra they put on !!! thinking they were doing good

Gg