How do Blackfyre partisans view Aegon IV? Do they gloss over his entire reign beyond the sword? Is he a case of ‘yeah he wasn’t that great but he knew who the right king was’?

We don’t know, tbh. 

Certainly, I think most of it comes down to the sword and Aegon’s comments about Daeron not being his son. For some of them, the Marcher Lords and Reachermen, war with Dorne might be part of it too. 

But I think for a lot of them, they cared far less about Aegon than they did about Daeron and Daemon.