update issues with macOS blackscreen

 Start up from macOS Recovery

To start up from macOS utilitRecovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… How to reinstall macOS - Apple Support

Command (⌘)-R

Install the latest macOS that was installed on your Mac.

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R

Install the macOS that came with your Mac, or the closest version still available.

2. Decide whether to erase (format) your disk

If you need to erase your disk before installing macOS, select Disk Utility from the Utilities window, then click Continue. You probably don't need to erase, unless you're selling or giving away your Mac or have an issue that requires you to erase. Learn more about when and how to erase.

3. Install macOS

After starting up from macOS Recovery, follow these steps to install macOS:

1. Choose Reinstall macOS (or Reinstall OS X) from the Utilities window.
2. Click Continue, then follow the onscreen instructions to choose your disk and begin installation.
3. If the installer asks to unlock your disk, enter the password you use to log in to your Mac. If it doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk. 

1. Please allow installation to complete without putting your Mac to sleep or closing its lid. During installation, your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.

If your Mac restarts to a setup assistant, but you're selling or giving it away, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.

If you never installed macOS Sierra 10.12.4

If macOS Sierra 10.12.4 or later was never installed on your Mac, macOS Recovery works differently:

• Command-R is still the recommended way to start up from macOS Recovery. This combination makes sure that the installation isn't associated with your Apple ID, which is important if you're selling or giving away your Mac.
• Option-Command-R installs the macOS that came with your Mac, or the closest version still available.
• Shift-Option-Command-R isn't available.

How to reinstall macOS - Apple Support 

Of course you may need to make a USB installer...

Create a bootable installer for macOS - Apple Support...

Create a bootable installer for macOS - Apple Support

Greetings DTBarker,

It looks like you joined the Apple Support Communities from an excellent starting point; Update macOS on Mac - Apple Support. However, if the screen is now showing as blank/black, the best steps to follow for this issue can be found here: If your Mac starts up to a blank screen - Apple Support.

1. Press and hold the power button on your Mac for about 10 seconds.* If your Mac is turned on, this turns it off.

2. Press and release the power button and see if your Mac starts up.

3. If your screen remains blank, press and hold the power button for about 10 seconds again to make sure that your Mac is turned off.

4. Try to start up from macOS Recovery and use Disk Utility to repair your startup disk. For details, see How to repair a Mac disk with Disk Utility.

* If Disk Utility finds no errors, reinstall macOS.

* If Disk Utility finds errors and repairs them, restart your Mac. If the issue returns after restarting, reinstall macOS.

5. If you can't start up from macOS Recovery or use Disk Utility, your Mac might not be turning on at all. In that case, follow the steps for if your Mac doesn't turn on.

6. If you're using an external display instead of a built-in display, make sure that the issue isn't with your external display. 

As it shows there, if you follow the steps and the problem remains, you'll want to contact Apple Support directly. You can reach out to them by clicking here: Get Support or over the phone by calling the number that is most local to your region: Contact Apple for support and service - Apple Support.

Best wishes!

I have the same issue, how can I just return to previous macOS? It's nonsense!!! My Mac is not working properly