I have also thought that PB backs typically looked black because the blue is so dark, but if you take a good look there are several variations to the color.
get-attachment.aspx-6.jpg
The majority of the PB's appear to be the darker blue as found on the example on the left. I have also seen a number of variations of Navy Blue as seen on the example on the right. The example in the middle appears to be a fairly uncommon variation. As you can see it is very light compared to the other possible colors. It is not a faded blue, but rather a distinct color. Several subjects exist with this much lighter shade of blue.
Shannon_PB_Back_SGC_40192.jpg
If anyone has any thoughts I would greatly appreciate hearing them.
Thanks,
Jerry