Canon BJC-3000 - Manual (Page 211)

211 Troubleshooting - Printout is Unsatisfactory
Backs of pages stained with ink
The backs of pages are stained after they are printed and ejected from the printer.
Check 1 Is the paper output slot dirty?
Check 2 Is a page ejected before the previous page is dry?
Check 3 Is greyscale printing being performed?