From my understanding, that's just the result of over processing the picture. It was probably a single pixel that was black, and when they "smoothed" out the picture, it probably made that shape. My guess would be that there isn't an actual notch there, but a darker region than what surrounds it. But we probably won't know for sure until we get a Pluto orbiter than can get close to these small satellites (so not for many hundreds of years).
From my understanding, that's just the result of over processing the picture. It was probably a single pixel that was black, and when they "smoothed" out the picture, it probably made that shape. My guess would be that there isn't an actual notch there, but a darker region than what surrounds it. But we probably won't know for sure until we get a Pluto orbiter than can get close to these small satellites (so not for many hundreds of years).