Oh man what a beaut!
Blasphemous is pretty much everything you can expect from a Metroidvania and more.
First, of course, that grim world is beautiful to look at, every new area is like a painting you're thrown into, and that horrific-religious (or religious-horrific) imagery is a total feast for the eyes. There's something very Baudelaire in that constant search for beauty and illumination in the most gory or derelict landscapes. That cathedral near the end absolutely blew my mind, and the same care has been given to every single enemy.
You could fear the game relies too much on looks only, and that happened with so many others it could be a legitimate concern, but that's not the case here: the interconnection of the different areas of the game has been very thoroughly planned to limit frustrating and useless constant backtracking. As you progress, you explore more and more of the world, but it's never a chore to revisit previous areas, and the checkpoints are placed in spots that always make sense. The game is challenging and you'll die a lot, but it still respects and value your time (as opposed to, say, Hollow Knight, which alway punishes you for dying).
So, yes, it's difficult, but in a good way, in a fair way: you'll always be on the edge of your sit, or your couch, because the platforming can demand precision, but mostly because enemies will never let you doze off. Every single one cause cause your downfall, so with each new screen you have to be actually focused and involved. Combat is extremely deliberate, there's no way to just casually cruise down from one boss to the other. And that's a good thing, because the fact that every single fight matters keeps things interesting.
Finally, the bosses are just great: they all have their style and design, all have very specific patterns... Apart from one or two, their feel very balanced, in a tough-but-fair way, and it feels very satisfying to beat them.
Anyway, nuff said, just get it, it's a masterpiece.