Blenders and food processors work differently. In a nutshell, blenders are better at blending liquids and food processors are better at chopping ingredients. And they are less likely to bruise more delicate ingredients like fresh herbs. For chimichurri sauce, I like to make it in my food processor since the overall goal is to chop. However if all you have is a blender, you can use it. It may not be "ideal" but it'll work and get the job done. But take note that one issue I have with my Vitamix high speed blender in recipes like this is that the volume of ingredients isn't quite enough that the blade can catch them and chop them. For that reason, you may want to double the recipe so that it's easier for your blender to grab ahold of the herbs and do its job.