In Tevinter, who is higher up the hierarchy, the Black Divine or the Archon?
Probably the Archon. In fact, in the old days the Archon was also the head of the Imperial Chantry. That power was reduced to a ceremonial role in order to appease the Orlesian Chantry. Ironically, it was a step towards the return of magocracy, since Chantry posts were some of the few still opened to mages in Tevinter after Hessarian's reforms, and they used it to keep their influence.
Also, Valhail, the first Black Divine, is said in the codex to have been "elected" by the Chantry in Tevinter, while in WoT it's said he was "appointed" by the Archon. In practice, it could be a combination of both: in many countries, the candidate for a given position can be chosen in a election but the actual appointment must be signed by another authority; it would mean that, at least ceremonially, the Archon is above the Black Divine.
Interestingly, WoT also says that Archons "rule by divine right", since it's traditional in Tevinter to believe that higher power and magic come from the gods (that it's changed from the Old Gods to the Maker is a secondary matter). In an attempt to pretend some sort of separation of powers, the Archon can't be a Magister or hold a rank in the Chantry.
However, the truth is that the Circles rule the country now. The Black Divine? Always from the ranks of the first enchanters. The Archon? Also an enchanter. The Magisterium? Several positions are held by the noble families (mages trained in the Circles), others are in the hands of the Chantry (controlled by the Circles) and seven are chosen among the Circles themselves.