Hi there! I understand you're seeing a black screen when trying to open the GHIN app on your Mac running OS X 10.15.7. To help you best, could you tell me if the app completely stays on a black screen after launching or if it happens only after logging in or certain actions? Also, is this the GHIN app downloaded from the Mac App Store or are you accessing GHIN through a web browser?

This info will help me guide you with the right steps. Meanwhile, here are some general steps for black screen issues on macOS apps that might help:

1. Force Quit and Relaunch:
- Press Command + Option + Escape to bring up the Force Quit menu.
- Select GHIN app and click "Force Quit."
- Then try opening the app again.

2. Restart Your Mac:
- Sometimes a quick restart clears temporary glitches.

3. Update the app:
- Check the App Store for any updates to the GHIN app and install them.

4. Check for macOS updates:
- Click the Apple menu > System Preferences > Software Update.
- Update to the latest available version compatible with your Mac.

5. Clear app cache and preferences:
- Though macOS apps store preferences typically in the Library folder, for GHIN app there might be specific cache files causing issues.
- You can navigate to ~/Library/Caches and look for any GHIN-related folders to remove (make sure to back up before deleting).

6. Reinstall the app:
- Delete GHIN app from the Applications folder by dragging it to Trash, then reinstall from the App Store.

If the black screen appears when using GHIN in a browser rather than the app, it might be related to browser JavaScript settings or a compatibility glitch. In that case, refreshing or trying another browser could help.

Let me know more about exactly when and how the black screen shows up, and I can provide more targeted instructions!