The screensaver starts for a moment and then the screen becomes black (Windows screensaver)
You probably need to adjust the monitor power settings as well.
When you set the screensaver idle time to lets say 5 minutes, make sure that the idle time for the monitor power off is set to something bigger, e.g. 15 minutes.
Windows will switch off your monitor after 15 minutes and you will have the screensaver running for 10 minutes.
The monitor power options are controlled from the same screen as the screensaver idle time.