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 of basil with the smallest leaves. They are the sweetest and most tender. Stack several leaves on top of each other with the stems aligned. Roll the stack into a “hot dog” shape. (Remember hot dog vs. hamburger folds from school?) Hold the roll with one hand and using your sharpest and thinnest knife cut straight down starting at the top of the leaves and moving down toward the stems. Slice as thin as possible. Don’t push on the basil, just easily glide the knife across the board and the herb. Delicately fluff the ribbons with your fingers to separate the ribbons. Place the chiffonade in a dry bowl and cover it with a damp paper towel. This is how basil can stay green for a few hours. It prevents discoloration and moisture loss.
A second method is to lightly coat the basil leaves with oil before stacking them and rolling them. The knife blade becomes coated with the oil that is on the leaves and as it cuts, the oil seals the cut. The oil provides a barrier so the air can’t get at the cut and turn it black.
Always remember, add basil at the very end of cooking. Do not add it in your pan as you are cooking or it will immediately turn black.
Now I know one more thing that can be done ahead of time which is always a good thing!