It might seem hard to believe, but Apple does not include a simple, blank screen saver with macOS. There are several workarounds floating around the Internet, from configuring the included Classic photo screen saver to displaying a blank black image to relying on Energy Saver to simply turn the screen off before the screen saver is shown.
We've solved this problem by offering this free screen saver. Simply download to your Mac, double click the file, and choose whether you'd like to install for just your user or all users of the computer. Then simply select Blank Screen from the list of available screen savers, and you're done!