I've just installed Ubuntu Server 18.04.1 (no GUI, X, nor anything graphical), and after breaking my back last night trying to turn off the screen I finally found the magic command:
setterm --blank 1
After executing the command the screen will turn off automatically every minute (if idle).
And even better, if you want the command to be executed automatically at boot, you can add it to the GRUB commandline, to do so we have to edit the next file:
sudo nano /etc/default/grub
Once there, just add
GRUB_CMDLINE_DEFAULT, it should look like this:
Then close the file and save it, after that just run
sudo update-grub and voila, every time you boot the screen will turn off automatically every 60 sec. (again, if idle).
And this way (adding the consoleblank to the GRUB) works even from remote terminals (ssh).