I think you're on the right track, there. If I remember correctly he says they are similar. I think Egwene ends up thinking about this as well, when she goes to study with them.
That is what I would generally think. I believe the assumption was that the "ageless" look is being linked to making oaths on the binding rod. So I'm just kind of figuring the Black Ajah secretly uses the Oath Rod.
What I want to know is how the Black Ajah members managed to retain the ageless appearance. They clearly have been unbound by at least the first oath, and by demonstration, the oath against using the Power as a weapon. Should we assume that, when someone becomes new to the Black perhaps there is...