How to keep basil leaves from turning black?
To extend the kitchen life of basil, treat it like cut flowers. Trim the ends, place the sprigs in some water (stems only, keep the leaves above the water line), and keep them in a cool location. It will still have a fairly short shelf life, so the faster you can use it, the better.

Do not refrigerate your basil, with or without trimming the ends; refrigeration causes the leaves to turn black and wilt. If you are not able to use all the basil in a timely manner, consider either keeping a small, living basil plant, or taking the time to chop the basil and freeze it with oil for future cooking use.