Blend on medium speed (see notes) until the pesto is mostly smooth. Add additional water, 1 tablespoon at a time, if needed to thin the pesto and help it blend.
Using pre-shelled pistachios can make the pesto a tiny bit darker as they're not as bright green as the ones roasted in the shell. If you have a high-powered blender, use medium speed. If you have a regular blender, you'll want to blend it on high speed.