Around here, most of the hulls are already squishy black, or getting there. I hulled mine by just twisting them in my hands wearing some leather gloves....of course I now have stained fingernails, but heck, I'm a guy, so who cares! I was then left with a mess of slimy black nuts which I cleaned by stirring them in a bucket of water a few times. I think people who get serious about black walnuts prefer to husk them before the husk starts slimming, so the nut comes out cleaner. I've read instructions saying to roll the green nuts across a knife and then twist them apart like you would an avocado. The nuts at that stage are still light colored and easy to clean up, I guess.
Carpathian walnuts are so much easier. They come out of their husks more like pecans. You can crack them in your hands. Of course, they sure don't taste like the black walnuts.
Chuck