This works, I tried it: for a typical 8×8″ pan of brownies, replace the eggs and oil with one pound of liquified (in blender) cooked black beans. Also add about 1/4 cup of water, since you won’t have the moisture from the oil and egg. If you’re concerned about sugar, replace with stevia, or maybe agave syrup, which has a lower glycemic index (I have not researched by how much).
I’ve also found that putting a layer of thin sliced newton-type apples on the bottom of the pan is a good thing. Spray the pan with something low caloric. Remember to have oven at full heat before putting brownies in.