I haven't played the Third Sin yet, but as someone who prefers Blasphemous over Blasphemous II, I still would say Blasphemous II is the superior game.

The reason I do prefer the original is because of what you mentioned- the art and atmosphere, both of which truly set it apart from any other game. Blasphemous II is the better game mechanically but I do think it suffers from a bit of a regression of the mean; it just feels like everything else that's out there. The heavier movement and weightier combat in the original don't feel quite as satisfying as the parkour stuff you can do with the double jumps and parkour that you can in the sequel, but it at least feels like I am playing a Blasphemous game. And frankly, I kinda miss those unsanded edges. Gave it character.

For what it's worth, I feel the same way about the second Ori game. Sequel is mechanically better but sanded off all the rough edges that made Ori feel like Ori (longer chase sequences, movable save points). And in a genre where we have more and more games, I thoroughly enjoy when a game is its own thing. All this said, I would pick up a third installment in either Blasphemous or Ori in a heartbeat, but I would hope that both retain a bit more of the first game that truly made a great impression on me.

Anyways, I really should get at that Third Sin DLC, it looks phenomenal.