I know there is marauders_slash, which seems to be the place where people post James/Sirius, and hprarepairs and variety_is are rare pair comms. I just did a search for Blackcest and saw two others, blackcest and hp_blackcest. There's also hp_cestfest, which is a fest and posting comm in one. And hp_slash and hp_adult, if you wanted something less specific. Uh, I hope that helps. :p

And thank you for reminding me that I need to read the fic! I love James/Sirius.