I agree in general with Walt's comments, although I have had good luck hand-filing a key notch for the keyed type. None have spun on me. I have used them in non-critical places where if it did spin, I can actually get pliers on them.
In this case, I am looking for a higher strength solution, and they really would be blind. Hopefully a one-time assembly, so as long as they don't spin on first tightening, all is good. The ability to install a flush, blind, nut would save me having to disassemble the part, which would involve screwing up paint, drilling out a ton of rivets, and then having to re-paint.