That tabard is a general acheivement tabard awarded for completing the Draenei campaign. The Blood Knight tabard is specifically Paladin only.
Meladria:
Its a quest for BE pally either at lvl 20/30. But it was before lvl squish so might be at lvl 15/20. You have to find them in old Undercity who’s gonna ask you to talk to the pally trainer in org. He will give it to you
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