A blender will make the bottom layer into paste before the top is touched. If you want to toss the paste into a skillet and caramelize it, that'll make a good sauce.

Food processor might be better, but still won't be even.

Source: I cook onions a lot, and am lazy. This article is great!

Always bust out the food processor when making soffritto or similar very small dice. Can do onions quickly and even with the method but carrots and others take quite some time.