Those are no longer big these days so no. Also, they're not going to restart a whole product category just for grapheneos.
As OnePlus is kinda dead and taken over by oppo, I'm guessing Sony. They have some similar collaboration in the past like with Jolla. My Sony XA2 was one of the few models that could run sailfish.
BlackBerry hasn't been OEM for their last few phones - the KeyOne, 2LE, and 2 were all outsourced to TCL, who is still making handsets. This would also fit with BlackBerry's security image, and even pull in the OnwardMobility vapourware.
I'm every bit as skeptical as you are, and in no universe is BlackBerry the OEM in question, but I would like to live in my delusion until GrapheneOS proves me wrong - I want a keyboard, dammit!
As OnePlus is kinda dead and taken over by oppo, I'm guessing Sony. They have some similar collaboration in the past like with Jolla. My Sony XA2 was one of the few models that could run sailfish.