Answer: There are several possible explanations, including that they have experienced low temperatures, they are suffering a fungal infection and need a brighter location with better air circulation, or they may have been damaged by insects. If your plants are outside, they would be considered annuals and are about finished for the year. If they are indoors, inspect them very carefully for any possible pests, looking at both top and bottom sides of the leaves. Basil is not a long lived plant, so after a while the plants simply lose vigor, become woody, and are somewhat more susceptible to problems. There is also a disease which can infect these plants and cause them to turn black, but this would include the stems. If the symptoms shows up on the stems, would discard the plants and start again, using fresh soil and carefully cleaned and disinfected equipment, including pots. I hope this helps.
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