OK, I found the manual still on my computer (I don't have the printer any longer):
Your product switches the black ink type automatically, depending on the type of media you load.
To disable automatic black ink switching, press the setup button and select System
Administration > Printer Settings > BK Ink Auto Change > Off.
For manual switching on the LCD:
Switching the black ink type takes several minutes and consumes some ink. Check the black ink type
media list to select the correct ink type for the media you loaded.
1. Press the home button.
2. Press the black ink change button.
3. Select Proceed and select one of these types of black ink to switch to:
• Photo Black to Matte Black; switching takes about 1.5 minutes
• Matte Black to Photo Black; switching takes about 3.5 minutes