frankiej:

Unfortunatley the whole lawn has now gone black including the grass,
weeds and moss...

Probably you had less grass than you thought. Weed n Feed shouldn't kill
the grass but the bare patches will look awful for a while until grass
fills in. Don't cut your grass too short and don't cut it in late fall
(depending upon where you live) and it should fill in nicely.

--
Mac Cool