I’m installing Windows 8.1, and after a reboot, it has a black screen.

This may be an issue with your display drivers. Try entering Safe Mode and updating the drivers:

1. When you boot your PC, try hitting the F8 key. This may take you to the Startup Settings that include Safe Mode.
2. If hitting F8 doesn’t work, and you have media for Windows 8.1 (such as a DVD or USB), you can boot from media using the instructions here. Further instructions about Safe Mode can be found here.
3. If you do not have media, use the power button to restart your computer during the boot process three times (do not let it get to the end of the boot process) . This should bring up the Windows Recovery Environment.
4. Once you have started in Safe Mode with Networking, check for updated drivers:
   
   

• Swipe in from the right edge of the screen, tap Settings, and then tap Change PC settings.
  (If you're using a mouse, point to the upper-right corner of the screen, move the mouse pointer down, click Settings, and then click Change PC settings.)
• Tap or click Update and recovery and then tap or click Windows Update.
• Tap or click View details, and tap or click Check for updates. (If you don't see View details, there aren't any optional updates available.)
• Under Optional, tap or click the update that you want to install, then click Install.

If no updated drivers are available through Windows Update, check the website of the manufacturer of your video card to see if they have updated drivers available.

If this does not resolve your issue, post in this thread.