I apologize if the discussion has ended, but I gotta chime in:
I can't possibly see how anyone could look at colored pictures of Rei and Ami from the manga (not the prototypes but using the final design) and think that they have black hair.
Ami's hair color is clearly a very dark navy in those pictures, and the presence of the violet/purple in Rei's hair clearly shows that the hair color is not meant to be black. If it's black, then it would not gleam violet/purple, nor would Ami's hair appear to be dark blue.
Rei's hair color is meant to be an extremely dark shade of violet/purple that could be mistaken for black in the dark.
It's definitely too light in Crystal, but Crystal is definitely doing its own thing in regards to its color palette.
Furthermore, you really can't use images from the manga itself to justify this because all characters with very dark hair colors are drawn with black hair since...you know...manga is mostly a black-and-white medium.
I'm not denying that Ami and Rei's hair colors are very close to black, but to say that they're meant to be black is taking it a step too far. Takeuchi was not always consistent in how she depicted colors, but the evidence is clear.