Basil can stay green for a few hours!
I have always used the chopped basil immediately because it turns brown quickly and then doesn't look appealing. Basil leaves are delicate and bruise easily. When you cut basil you are creating surfaces to be exposed to air. The basil turns black because it is oxidizing, just like an apple. You cannot stop this process, but you can slow it down.
Using a chiffonade cut is one way to slow it down. A chiffonade cut makes the basil into ribbons. How do you create the ribbons? Choose sprigs ...