Blasphemous crossover?

They discussed it with 505, yes. I assume Artplay and IGA are co. are fine with it as well, they would have to be--Blasphemous is being published by a decently respected name in the industry (Team 17) and they wouldn't just do this without making sure the Bloodstained team was okay with it as well. Question has mentioned that they (505) were working on this crossover for quite a while so it's definitely not a "liberty."

As for how it happened? Not exactly pure chance...I mean, it is coincidental that Blasphemous and Bloodstained came out within a relatively close span of time, but as it happened there's a significant overlap in the fanbases of the game, myself being a prominent example. A while ago I commissioned a fellow named Turbogum to post some artwork of Miriam and the protagonist of Blasphemous (The Penitent One) doing a brofist. The official Bloodstained account retweeted that on Twitter, and people loved it. Now, that was just a fun little fanart thing, so nothing to be taken too seriously, but I assume after looking into it further the marketing analysts for both publishing companies (505 and Team 17) concluded that there really was a lot of folks who would be into both games if they hadn't already bought both, so after a while they did a sword and whip bundle sale on Steam because so many folks liked both games. That worked out really well so I suppose an official crossover was the next logical step. It's also important to note that the devs of Blasphemous loved Symphony of the Night; one of the devs on Twitter said he always wanted to be "metroidvania bros" with Igarashi. To say he got his wish would be an understatement, it seems