If you have multiple displays and wish to turn one of them off, you should use Xfce's display settings dialog for that.
If you use the
xrandr command while Xfce is also controlling the display settings, Xfce might promptly restore the state configured for it, overriding your
xrandr command - which would match what seems to be happening to you.
If you have just one display, turning it off with
xrandr would leave the X11 server (and your GUI session) with no functional screens, and it won't like that at all. It will re-detect screens, "find" your screen again and effectively override your command.
To enable the screensaver, you should rather use
xfce4-screensaver-command --activate or similar. Typically, this first just turns the screen black, then (optionally) enables a DPMS power-save mode after a configurable number of minutes.
Since letting the screen power down completely makes it slower to power up again than just making the screen black, there is usually a delay of a minute or two after the screensaver starts before a power-save mode kicks in. That enables a faster response if you are e.g. presenting a document and the screensaver accidentally kicks in while you are talking.
(Actual presentation applications usually override the screensaver when in "presentation mode", but perhaps you're presenting something using a plain Web browser or some other program?)