DIAGNOSIS Yeah, this is a common complaint. The Hub doesn't have a true "off" setting for the screen in its standard menus, just various levels of dimming and an ambient mode.
FIX You need to set up a Routine. Open the Alexa app, go to More > Routines > Create. For "When this happens," choose Schedule and set your bedtime (e.g., 11:00 PM daily). For "Add action," choose Smart Home > Device Settings. Select your Echo Hub, then choose "Display Brightness" and set it to the lowest possible setting (1%). Add another action right after: choose Smart Home > Device Settings again, select the Hub, and this time choose "Turn Off Display." This sequence will black it out.
IF THAT DIDN'T WORK The "Turn Off Display" action might just put it in ambient mode. For a nuclear option, get a smart plug. Plug the Hub's power adapter into the smart plug, and create a Routine that turns that plug off at night and on in the morning. The downside is a ~45 second boot time when it powers back on.
HEADS UP If you use the smart plug method, any automations or commands that require the Hub will fail while it's booting. Don't schedule the morning power-on right before an important alarm or automation.