Try comfrey if you want to stabilize the slope. Comfrey grows vegetation profusely, it can be chopped pretty much at will for all the mulch or composting material you want. Or you can just leave it to its own devices. It can become invasive depending on your climate, the site, and the variety. Thyme, prostrate Rosemary, common sage, and oregano are good on slopes and will flower. Vinca minor is a ground cover that spreads readily and will flower. Moss rose (portulaca) is another flowering low growing fast spreader. Creeping phlox, also good but I haven't found seed for it yet.
Dwarf forsythia (prostrate form) or flowering quince. Prostrate sumac. Lilacs. I love lilacs.
I don't know of any particular reaction to black walnut for any of those, but it may be so. Some might not be appropriate for your area or conditions. Don't know where you are or what kind of weather you should expect. There are other things that might work but the above should give you a good start.
