Betty crocker’s box mix has only vegan ingredients. To the mix, you add a 15oz can of rinsed black beans,( open can, but keep lid on. Running water into can, rinse as much of the liquid off as you can. Then add water back to the can up to the top. Dump into food processor or blender and purée) to which water has been added to the top of the can. Purée the beans, then add to the brownie mix. You can add walnuts, etc. Bake as per package instructions.
I’ve made these and I enjoyed them as did other people. I have to wonder if anyone farted later on, though, lol. I never told people, except for close friends, that I’d made them with black beans.
Variation : add a cup of vegan oreo cookies,( newman’s own makes these) crushed, to the batter before baking.
Thanks again for sharing. 
Keep sharing more with us. 