This looks like it could be the start of some Basil Downy Mildew. If you look on the undersides of the leaves you will likely see the gray-purple fuzz that is indicative of this pathogen.

Unfortunately, this is a serious pathogen of basil. It's recommended that you harvest the affected plants (they are safe to eat). To prevent future issues with this, it's best to use steam-treated seeds or plant resistant varieties. More information on this topic can be found here.

https://hort.extension.wisc.edu/articles/basil-downy-mildew-2/

https://extension.umn.edu/disease-management/basil-downy-mildew#:~:text=Susceptibility%20of%20basil%20varieties%20to%20basil%20downy%20mildew&text=If%20any%20plant%20has%20basil,after%20rain%2C%20dew%20or%20irrigation.

If you do not see the fuzziness on the undersides of the leaves, this may simply be some botrytis or other less serious pathogen. You could simply pinch the plants and remove the impacted leaves to prevent spread.