What a beautiful kitten!
Yes, I do think it is unusual, and probably rare, but I would wait until the little one grows up before deciding how rare.
Sometimes, black kittens will show their tabby markings and as they mature, their coat becomes all black and hides those markings.
I hope that doesn't happen with this kitten, that would be very striking! Take a look at the hair coat, part the hairs in the two different colored areas and see what differences you see. If the hair shafts are all one color from root to tip, the kitten may become all-over one color as it matures. But, if the two different colors show a difference, like one (the dark area) is solid from root to tip and the other (the lighter area) has different shading or bands of color, it may keep its distinctive markings as it matures.
Please, take lots of pictures and t us watch as this kitten grows up! I would love to see how it turns out!
Also, what have you named her? I'd love to call her 'mine'!