This problem is usually caused by corrupted user map or save games.

To resolve this issue please do the following -

1. Backup and Save Games and User Created Maps you have in your Documents folder.
The default location of this folder is usually located - 
C:\Users\<USER>\Documents\Stronghold 3
2. Disable the Steam Steam Cloud.
This is located in Steam.  Click on Steam, Settings, click on Cloud.  Untick 'Enable Steam Cloud synchronization for applications which support it'
3. Delete the Stronghold 3 Documents folder.
The default location of this folder is usually located - 
C:\Users\<USER>\Documents\Stronghold 3
Please ensure you have Backed up your files before doing this.
4. Run Stronghold 3 as per normal.
If the game is running and your can open the Blackstaff Campaign.  This means one of your user Save games or User created maps has become corrupted.  If you need Saves or Maps you may want to move them from your Backup folder to the Default location, one by one and if the Blackstaff Campaign stops working again.  Remove the last Save or Map that was moved across.

If this doesn't work the Blackstaff Campaign doesn't work.

Please delete Uninstall Stronghold 3 from Steam.
Select the Stronghold 3
Manage -
Browse Uninstall
Delete any folder or files left behind.
Select the Stronghold 3
Manage -
Browse local files
The Default location is here -
C:\Program Files (x86)\Steam\steamapps\common\Stronghold3
Reinstall Stronghold 3 in Steam.