That's clear enough. From the images I've seen it's got to be primary transfer belt cleaning or black drum cleaning.

If it's primary transfer belt cleaning the color of the background will be composite black: closer to brown or purple.
If it's black drum cleaning the color of the background will be black ... just black.
I can't really be sure from the scans, but Colorpic tells me the background is ~6%K and nothing else. =^..^=