Why do the two hand builds not pick up Black Ice?
Scourge Strike is an ever decreasing portion of our total dps. With 4p t9, Dark Conviction becomes that much more attractive. Subversion is a mandatory talent now, so even if you did pick up Black Ice, you would still have to shove points into the Blood tree. So on. The bottom line is: Black Ice just doesn't do as much dps. There isn't much more to it than that.

Why is Obliterate no longer used?
Obliterate does, in fact, still out damage Scourge Strike on an individual strike level. However, with Dirge no longer affecting Obliterate, Obliterate builds generate substantially less runic power, and thus fall further and further behind, despite ArP scaling and the like.