In case anyone was still concerned with Blood DKs in Challenge Modes, the first group to clear Gold in all the 5-mans had 2 blood DKs.
http://www.mmo-champion.com/content/...-MoP-Wallpaper
You can also see that a bunch of the top challenge mode progression rankings are held by DKs (not sure if blood or not).
http://www.wowprogress.com/challenge...ss.deathknight