Turbine D
I did check and also amplified those pictures in the ATSB report to 800 x the original size, beyond pixelling starts and there is too much noise.
I must admit that I cannot say wether what one sees in the area you mentioned but also at another one in the lower left base is a dark deposit or just shade. Unfortunately it looks like the picture was taken just off axis, slightly right of the center of the cross section. So either is possible.
However, I also checked for pictures of the disk piece in earlier posts. Check the post nrs. 42; 84 ; 587 ; 680 - they show several aspects that has to be considered. In post 42 is a picture where police officers carry that piece by hand and using some questionable dirty pieces of cloth for that. Post nr. 84 shows that part probably in a private car trunck before or at delivery to the police. Although these pictures show a non professional handling of that important part some kind of deposit remained untouched to some extend.
Post nr. 587 shows that same part on a small lory, also only the rear side of the disk. What is remarkable there are still some darker areas visible may be some kind of deposit ???
Finally there is that false colour picture in post nr. 680. It too shows some darker areas, even in false colour.
What I also have realized is a difference in surface colour of those early pictures compared to the "official" one in the ATSB report. Explanation for that - either the different colours come from repeated treatment, handling and reproduction of the original pictures or the disk was cleaned, hopefully after samples of any foreighn material on the surfaces was secured,
A pitty, I have to say it again, that there are no better and more detailed pictures at hand.