I'm trying to help one of my daughters solve a printer problem...black
ink smears on her card stock.

She prints wirelessly from a Windows 7 laptop to a Canon MG 6120 printer.
It's printed fine for years, but it's started to leave (only) black ink
smudges on one edge of the 4"x6" card stock that she prints on, as shown
here:

https://postimg.cc/t1nxV6tP

The card stock is placed in the rear tray. We've tried cleaning the
paper feed roller, cleaning the pad in the cassette, both per the
manual's instructions. The smudging remains. I sense that something has
wet ink on the transport path that needs cleaning.

I remember many years ago, reading on this newsgroup, about 'overflow
sponges' that absorb ink when certain maintenance procedures are
performed (not necessarily about this MG 6120), such as head or cartridge
cleaning/alignment.

I don't know if the MG 6120 even has an absorbing sponge, but thought I'd
at least mention this in case someone knows this may be the issue. By
the way, these cleaning/alignment procedures have been performed, and all
is perfect.

Twenty-weight paper prints fine. I'm not sure of the card stock weight.

The manual does not address this problem.

Any ideas?