Gen is right (Matthew, WHATEVER...) in the sense that fitting in the Kyurem formes can be challenging. It's not just the SR weak - it's the SR weak combined with its Spikes vulnerability. This weakness plagues Reshiram quite heavily, and Kyurem doesn't take it any better. However, if you do manage to fit one in, you will be rewarded quite nicely (moreso with Kyurem-W than Kyurem-B, though). To this day I still can't get myself a comfortable team with Kyurem-B, and only one with Kyurem-W.
But enough of that. This thread is about Kyurem-B.
Having played with Kyurem-B (though with rather uncomfortable teams), the mixed set is definitely the most potent, as it can fully utilize its raw power and coverage and batter down falls with ease. syrim covered pretty much everything so i don't think I need to add much more to that. Another significant trait is its neutrality to ice / resistance to water and electric, which makes it a really good backup Kyogre check. Being able to handle the king of Ubers combined with its wallbreaking strength grant it a couple of switch in opportunities (despite the weaknesses attached to its Ice-type), and cause some havoc.