I picked a bunch of basil last week so we could put up some pesto.

I brought the leaves inside, washed them, and set them on the counter in the kitchen while gathering other ingredients for pesto (grating cheese, etc). Many of the leaves started to turn black -- especially at the tips and in the veins. It didn't seem to affect the quality of the end product.

I'm planning to dry the next harvest batch, and I'd like to make sure it doesn't turn black during drying.

* What makes basil leaves turn black?
* What should I do post-harvest to prevent this?