Basil leaves can turn black when stored in the fridge or below 45 degrees.
To store for a few days: leave basil in a plastic bag on the counter until using.
To store for a full week; store basil with stems in a small glass jar and cover with a plastic bag, on the kitchen counter.
Basil bruises extremely easily. If serving immediately you can cut with a knife. If not serving immediately prevent blackening of the leaves by:
-coating/tossing chopped leaves in oil
-ripping leaves rather than chopping with a knife
-blending with oil and salt
When using as a garnish you can make the dish ahead of time and add the basil before dishing up the meal.