I've tried the default keybind while holding a blueprint, nothing happens. I tried binding a secondary button and it doesn't work. The button on the toolbar appears disabled. This doesn't seem to work. I've got SE/K2 installed, but even a basic belt blueprint doesn't work

I think it only works if the blueprint is in your inventory (nav view inventory from SE also works).
You cannot convert a blueprint which is from your blueprint library directly (neither in game library nor your global library).

Might this be the issue for you?

Yes, I'm sorry I forgot to follow up. The keybind works, I was just using the wrong one. You can lock this thread, I did a big dumb. :)