I don't think the Champion is subject to the Blood Oath, really - I don't recall ever giving it to Thrall, Garrosh, Vol'jin, or Sylvanas. The Blood Oath is only really ever leveraged the once, in inducting the Taunka into the Horde. It may not be a common instrument of Horde diplomacy, or it may be unique to Overlord Agmar's particular methods of outreach.