Evergreen Feed & Weed

On saturday i put some feed & weed on the lawn, it then rained a couple of hours after and most of yesterday afternoon. I have noticed large black patches forming where the main consentration of weeds are bit it seems to be turning the grass black as well.
Is this normal and it will green up in seven days?
Dan x
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It could be killing moss in the lawn, That's possibly why there are black patches