This is Ethan's back sprite. Clearly black. This is his actual sprite, and this is Dawn's sprite. Same colours.

You don't really need something as fancy as gimp to check colours. You can use paint. Anyway, using Win7's paint, the colour for Dawn's Sugimori art for Platinum yields black, and I checked in more than two places. I basically checked wherever the colour changed. It corresponds with what a visual inspection tells me. You know what else coresponded with a visual inspection? That Sabrina's Gen I work (the head shot from the manual) had green hair.

Yeah, sure when you look at where the cursor is, its on the blue end of that spectrum going horizontally, but guess what? There's no black on that colour spectrum. Technically there isn't a black colour when you refract light. But notice how the cursor is down there where the hue practically becomes black? Yeah, its because they're going for black. In any case, notice how the colours when analyzed give a number for red, green, and blue? (Yours has them too as R/G/B). Blue is always higher on that spectrum. There's a reason for that. You get much more naturally black hair tones when there's more blue than the other colours. Just screw around with the cursor on paint, keep the vertical llength of the cursor the same, toggle it left and right, and you'll notice that having less blue makes the colour look different. Going towards the red spectrum actually makes it look brown. Not really going to post pictures since this is something everyone can do. Red, green and blue are what we call the additive colours, there what we use to make every other colour we see in our eyes. They used more blue to make that black shading effect.

Now this transitions us back into the sprites. They don't have as much colour at their disposal as the official artwork. We've shown that the black hair colours they use compose mostly of a bluish hue over red and green, so obviously what colour are they going to default to when they need to give their characters more hair tone?

We'll call what we did there examining hair colour on the micro scale. On the micro scale, her hair is clearly blue. But on the macro scale, her hair colour for her official artwork is black. What appears to our eyes naturally from the Sugimori artwork is that it is black. Her anime chose blue, but again, the anime themselves don't seem to have as much colour working for them compared to the still pictures of the game's official artwork (and the anime change Volkner's hair a bit). Dawn's hair in the Sugimori art has like three different shades visible to the naked eye. Her anime has like, two, now put on top of that the already black clothing.

Also Lucas has the same back sprite hair colour. Norman's manga rendition also has blue hair.

It's obvious this series substitutes blue in for black a majority of the time.