When I don’t have time to make my basil pesto, I freeze basil leaves in their own large ice cubes. I’ve done this a couple of times, and it has seemed to work pretty well. I just put basil leaves into plastic dredging dishes, fill with water, and freeze. It’s important to cover the basil leaves with water. If any come in contact with the air, they will turn black.
Once the block freezes, I pop it out and put it in a zip-top bag.
When ready to use, I just lay the basil block in a strainer in the sink. I have to use the basil almost immediately after they defrost or else they will lose their flavor and go bad quickly.