While I guess you ask because you want to know if the story of the first is important to the second and I can't answer that because I only played the first.
But
If you like hard but rewarding Metrodvania games and you are interested in the weird and interesting religious/horror pixel aesthetic I say: definitely play the first Blasphemous!
If this interests you, no reason not to play the first! I personally bounced off of it, but I don't think that is the game's fault.
The game's combat strikes a unique balance between "slow and deliberate" (ala Dark Souls) and traditional fast-paced platformer combat. I went in wanting the game to feel like Hollow Knight, and it is definitely not the same. You don't play a bouncy lil bug man, you play a big heavy knight. Big heavy knight + platforming felt like a weird combo to me, so I put it down. But not every game is for everybody!
I think the only thing I might have changed about the first game is removing relic slots. It felt a little needless to limit the number of keys/exploration upgrades the player can use at any one time. Other than that, solid game.