@moira@mastodon.murkworks.net I'm not sure this was intentional / intended to be used; There are multiple black palette entries in the NES PPU, and analogue video at the time was a wild west of sorts despite there being standards.
... Mostly because the US and Japan had different standards for what "black" should be, and the PPU wasn't changed to reflect that between regions. Japanese games since 1985 are usually set up for black to be at 0 IRE (NTSC-J), whereas US standards were 7.5 IRE (NTSC-M, meaning "reference black" was higher than 0V on the wire). I don't recall what the reasoning was now, though I *think* it was for broadcast signal integrity/reliability.
I'm not positive that this is why 0d is like that, though, I think it's just how the hardware operates since it generates a composite video signal *inside* the PPU, rather than RGB. Many games did use the colour, including western ones.