Black stuff on basil?
This looks like four lined plant bug damage.
I would cut away the damage and use the rest.
It could be bacterial leaf spot, but leaves need a look under magnification to confirm it. You can submit samples to https://pestid.msu.edu for exact diagnosis.
Several diseases to watch for are described here-
Pruning back the basil will stimulate new growth. Prune back about two-thirds of the plant and fertilize so the plant will put on new, healthy leaves. Pruning will also force new branching and a stockier plant. It’s best to water in the morning at the base of the plant, keeping the foliage dry, to prevent some disease problems.
Consider using a net over plants to prevent insect damage. Net houses can be purchased at garden supply stores, or you can make your own with nylon tulle( fabric stores) or floating row cover fabric around poles, tomato frames, etc. and firmly fastened at the base so insects don’t crawl under. Gather fabric at top so you can open it to harvest, or unfasten one side and fold up to reach in.
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