The short answer is that the driver should only use photo black when printing on glossy or semi-gloss papers. The reality is that this may or may not happen and there is no way to tell what ink the Epson driver uses.
The purpose for photo black is to have a black ink that works on glossy paper. The formulation of many pigment black inks cause a lack of gloss wherever there is a black dot. This causes odd effects when you look at the photo off axis that gives the photo odd iridescent effect and changes the photo depending on what angle you look at it from. Photo black is a different, usually dye-based, formulation that doesn't cause this effect on glossy paper.
As mentioned, and as originally written, drivers are really only supposed to use photo black when you tell the driver that you are printing on glossy media. That's the whole purpose of telling the driver what kind of photo media you are printing on. However, Epson isn't in the business to sell printers. They are in the business of giving away printers and selling ink. They learned that they could tell the user that a "feature" of the printer was to use the "photo" black in combination with the regular black for deeper, darker blacks when printing on normal paper. They do this because it more quickly depletes the more expensive photo black and makes people buy more ink.
When they first started this, many people recognized it as the questionable practice that it was, and so by popular demand drivers had a way of turning this off. Epson wanted you to believe that turning it off reduced the quality of black, so they labelled the setting "high quality black". Most people who learned about it still turned it off though. Epson didn't like how much it was turned off so they have done a couple things depending on the printer and driver you have. Some drivers have removed the setting altogether, and the driver just always mixes the two inks and there is sometimes nothing you can do about it. On printers and drivers that still have the ability to turn it off, to "encourage" (read "strong-arm") people into leaving this setting on, when you turn it off the driver also reduces the amount of regular black that is dropped on the page. This has the effect of actually making the black look a little crappier when you turn off high-quality black.
So, comb through your printer driver settings, including all the advanced settings, and look for "high quality black" or something labelled similarly. If you see it you can turn it off and hopefully the driver won't deplete your photo black cartridge. On some printers, you can actually physically remove the photo black cartridge and the printer will still print. Don't do this for too too long, though, because you can dry out the ink feed.