Is the Blood Knight Tabard still available?

That tabard is a general acheivement tabard awarded for completing the Draenei campaign. The Blood Knight tabard is specifically Paladin only.

Is it moggable for other classes, or Paladin only?

Fun fact: Thanks to the Darkmoon Faire, my Lightforged Draenei warrior can mog the Lightforged Paldin set.

Turn on low level quest. should be easier to find it