I agree with the black gravel! I think it REALLY looks good with live plants. It brings out the colors in everything.
Also, I've read that bettas like darker colored gravel because the lighter colored gravel reflects light and can be very bright. I'm not 100% sure on that, though. But even when I've read about very bright lights causing blindness, lighter colored gravel doesn't seem to help.
I have white gravel in three of my tanks that I would like to replace with black gravel. I just think it looks nice, too.
I wouldn't be concerned about it leaching anything as long as you rinse it well. Soak it for awhile if you're concerned. Anything will probably fade just because of use and being exposed to light, but it doesn't mean it will leach anything.