I had to learn this the hard way too. Never, ever manually delete (or add, or modify) files in
Blue Iris managed folders. It's a certainty that this will damage Blue Iris' fragile database. At this point, I would:
1. Close the Blue Iris app.
2. Stop the Blue Iris service.
3. Delete the *.db files in the database folder.
4. Reboot.
5. Start Blue Iris and let it regenerate the database.