The word giallo is thrown around in a lot of the reviews here - and not least of which in the description on MUBI - but it strikes me that Yann Gonzalez isn't necessarily all that interested in getting some shocks or indulging so much in the kill set pieces (not that he doesn't completely, with one involving lots of 360 degree pans revealing in each succession the killer approaching and then slicing away) as much as he is in pushing the colors that hes working with and mixing film stocks and, in his way, doing a meta comment on using art as a way to fight back.
When Vanessa Paradis's Anne goes to a police station to be briefly questioned about one of her actors being offed, this is then cut away to her recreating this with her own actors (Anal Fury 5 quickly becomes "Homocidal," the best pun you never thought of because why would you), and when she thinks she can draw out who may be the killer, she quickly stages a scene of sado-masochism... And gets what she is asking for (in the one scene that is truly suspenseful). What I'm trying to say here is that if you go in to Knife+Heart expecting a usual Argento or Fulci or one of those directors, you'll be not so much disappointed as thrown off.
And yes, MUBI, it is "unapologetically queer", which, you know, good. But it is also unapologetically French: the Italians had their own method of madness when it came to drawing out violent and/or surreal set pieces (one commonality is a lush and vibrant and spine-tingling score), and this has some surrealism as well, like with the black and white 16mm that feels like it's deliberately cut in from another movie.
But it also embraces and in fact demands that it be erotic and push the limits (albeit no actual genitals are seen, they might as well be), and Gonzalez is in love with color in a particular way. When we see red, it feels especially red and fiery; when we see blue, it's particularly somber and sad. And black? Well, that's the name of the game, man/woman - darkness is all around these characters, but what I also find striking is that, for the types the gay actors and some crew are, they feel like real people, which I often didn't get from Italian Giallos.
One issue though is that it is a director preferring style over substance. He loves Paradis clearly and what she can bring, but her role is thin and I never really felt for her (though she is, without spoiling, denied her moment of redemption that should come). Maybe that makes her more tragic, but I just didn't feel it, and that is what also is more French to me than anything - the sense of doomed romance and ennui which... Cool. But it's definitely more of a visual and sensory experience than one for story or real pathos.