I was mulling this one over, so I took a look at Harold McGee's chapter on browning in The Curious Cook. He says that lemon and other acidic additions don't work (and make the pesto sour). He doesn't like the blanching method either, as the aroma is diminished. He recommends being sure to take out the stems and stem bases, using pine nuts rather than walnuts, and using some lemon or cream of tartar in your pasta (if that's what you're serving it with).

It's a great chapter in a really good book.

did you add any acid, like lemon juice? that will help keep it from turning brown. i never tried the blanching trick, but i also add plenty of lemon juice and olive oil to my pesto and have not had any problem with it turning brown.

I read somewhere that if you make certain the basil is totally dry (no water droplets) prior to processing, the leaves will remain green. I tried this last year and it worked!

Basil is pretty delicate, and it turns brown easily. Some people put a layer of olive oil on the surface to guard against it browning. But that's to keep the surface green. It won't guard against a whole batch. I've never tried that blanching trick, which seems contrary to reason--but I've heard it works.

You need to quickly blanch the basil in boiling water and immediately chill it in an ice bath. This will keep it green. Good luck!