I prepare the black beans and then mash them up a bit and cook them with hamburger as I'm browning it. Then I use the mix for anything I use ground hamburger for (spaghetti, tacos). The kids don't complain because they don't know they are there at all.
I have also added fully cooked black beans to meat loaf and that's really good to. I also use light color beans and hide them with cooked chicken dishes.