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There are actually two different variations:
black & a very dark slate gray,
and black & a very dark/nearly navy blue (hence why, in some early tale or other, Selina's message to B in dropping by her apartment specified 'wear the blue')

The reason is simple: one solid mass of color, even black, is not as effective at camoflage. Breaking it up with another very dark color enables B to blend into the shadows in that very creepy fashion.

Bruce claims the two variations (blue and slate) are because the gray is more effective at street level, while the blue works a little better swinging around the rooftops where there is more city-glare. Personally, I think he's making it up because the real reason is un-batlike and he doesn't want us to know.