Does Nightmare Fist work with the Black Seraph stance Walk in the Dark?
And do I get a new stance at lvl 8, or will this be errataed to 9, because I remember reading this somewhere...
Personally, I don't see a problem with them working together. I'd allow it, at least.
I'm having a hard time seeing how you could justify this not working.
Black Seraph abilities are supernatural by default, fulfilling the requirements of nightmare fist easily.