Lion should not cause the blacks to print faded. The printer drivers used by Snow Leopard and Leopard are the same version as that used by Lion.
For starters I suggest you check that you have the latest version installed. This Apple KB article shows the MG6100 series has v10.51.2.0 as the latest version.
I also recommend you do a Nozzle test print. This can be executed from the MG6120 control panel or via the Printer Utilties window (via Print & Scan, select the printer and then open Options & Supplies and then select the Utilties tab).
If the nozzle test print is okay but the blacks still look faded when printing photos (note the printer has two black ink tanks - one for photos and one for text) then try a reset of the printing system. This will remove the MG6120 from the Print & Scan pane but not the supporting drivers. So you will only have to add the printer again.
Thanks. Your suggestions led me down a path that solved my problem. When I did the nozzle test print, black didn't show up at all. All of the other colors and the black grid printed correctly. I contacted canon technical support and they had me reset the nozzles by removing the ink cartridges and the ink cartridge holder then reinstalling in reverse. Problem fixed! Thanks for your help.