#3 There are more potential feuds for him as a heel on SmackDown
Even though the rosters of both Raw and SmackDown are relatively equal in terms of faces and heels on each show, if Black becomes a heel of some kind instead of the face he's portrayed, he'll have plenty of faces to feud with.
While the likes of Kofi Kingston and Ali were already members of SmackDown, both Finn Balor and Roman Reigns joined the blue brand during the shake up. In terms of big-time heels, there are really only Orton, Elias and Kevin Owens at the moment. Andrade could also join the upper echelon if given the chance, but Black would likely have more potential initially if he feuded with the aforementioned faces of SmackDown.
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The quality of the matches would never be in doubt, but if WWE wants to paint Black as someone to be reckoned with, toppling Kingston, Reigns or Balor at some point would likely get him over. And that's not to mention the ability of guys like Kofi, Ali, Finn selling the Black Mass kick.