If you blend the olive oil, even though we said not to, instead of mixing it by hand, you run the risk of a bitter pesto. If your kale pesto tastes bitter, you can add a little more parmesan cheese and a pinch of salt to the pesto, then stir it together -- by hand. You could also add a tiny bit of baking soda to the pesto to take away some of the bitterness. Be careful to only add a pinch at a time and taste it after you do because you don't actually want to taste the baking soda.