Have the exact same issue, and figured it out. I have a laptop that sits on my desk that I use occasionally, and is always on, so I want the display to go OFF after one minute, not blank. In XFCE 4.8 (Newest at this time), I disabled monitor power management in the XFCE power management settings and enabled the advanced options in the screensaver settings. There is a section for power management. Setting 0 for the first two options disables them, and setting one minute for display off turns it off after one minute.
I was enabling display off in power management, and my display's backlight would come on when the screensaver would come on, and wouldn't turn off if I disabled the screensaver... Quirky.
You'd think XFCE's team would fix something like this, but at least this works.