Here are some marbles with (what some would consider to be) Oxblood (while maybe others wouldn't). It's pretty easy to see, I think, why anyone might call Akro's Oxblood, "true" oxblood (or MFC's, but I don't have any of that anymore).
Included in the second picture are: MK, Vitro, Peltier, Ravenswood, Champion, Alley, and a German Onionskin