There isn't any pictures showing underside of rear fuselage. But this kind of sooting is not normal for sure.
It points to a fire after initial event and for quite some time. It means that after initial event tail remained attached for quite some time.
Edit:
Actually this sooting is on the roof. Right where vertical stabilzer used to be. It could be oil. From pictures you can see, that this kind of "sooting" has been almost constant throughout this plane life. As there is no obvious sooting on a tail part roof, this is not an indication of fire around fuselage.