The black 3DS XL is being advertised with Mario and Luigi: Dream Team (which I immediately thought would be green, but I recall there being a special Luigi 3DS XL in Japan so we're probably getting black instead). I suspect black will be a new color sold alongside (possibly replacing either red or blue) rather than a special limited edition thing like the Animal Crossing 3DS XL.
Still, if Nintendo wants to play around with different colors, I think it would be beneficial to them to always have a neutral color like black or white (even Silver). Like you, I would have already had an XL if black was available from day one. I may have considered blue if it was a darker shade. Oh well.