This is how I've done this.... Basically add a camera, and use 'Black-out'...
1. Copy a camera.
2. Go to... Trigger/Motion Sensor/ Configure...
3. Check the box 'Black-out masked areas'.
4. Check 'Use zones and hot spots', and click 'Edit'.
5. Click the 'Clear' button to clear and green areas.
6. Click 'OK' 3 times until you exit.
7. The camera display should be black.
8. Setup Overlay.
Now you can setup 24/7 recording with overlays on a black background. CPU is minimal for the copied camera. Overlay is extra!
Update:
Further testing I discovered that the 'Black-out' area is not entirely black. The image shows thru just a bit.
I instead added a black image I downloaded from google images, and stretched it to fit the screen.
Do thing this you'll find you' will need to continually push this image to the back to work on the other overlays, and, finally to see all overlays.
I've found a better way to add a blank camera. It has been mentioned, but I thought I'd correct my previous post.
Basically, add a new camera as 'Screen Capture' with 'Blackness', at 1fps. It will use much less resources and produce a much smaller bvr file.
1. Click 'Add Camera"
2. Specify the Full & Short name.
3. Select 'USB, Analog, other'.
4. Uncheck the option 'Enable motion detection'.
5. Click 'OK'. The Properties window will appear.
6. In the Video tab, select 'Screen Capture', then, in the drop down select 'Blackness'.
7. In 'Image Format' set the size of the display. I set this to 1280x960. It matches my other cameras. Setting it higher will give better resolution if you also use radar or other images. However, if your only going to use the BIT overlays, set this low, like 640x480.
8. Set the fps to 1. You don't need it any higher. If you don't have a time overlay with seconds, this can be every 5, 10, 15 seconds...
9. Enable 'Text and Graphic Overlays', and add your BIT overlays. (This can be done last after you complete these steps)
10. Click the 'Record Tab'.
11. Select 'Continuous'.
12. Pre trigger should be unchecked.
13. Check 'Combine or cut video', and select a duration. I use 6 hrs to match my cameras.
14. Click the 'Video file format and compression' box.
15. File Format: Make sure 'Blue Iris DVR' is selected.
16. Video Compression: Select 'Re-encode with settings'. Use the default settings. This is required to record the overlays.
17. Click 'OK' a few times to exit back to the camera.
18. Make sure you export the camera, and BI settings.
19. Adjust your BIT overlays as needed.
With the BIT text overlays, and 3 image overlays - one being the NWS local radar image, the bvr file size per 6 hrs is only between 473-500meg, or, about 979meg a day. This would be much less at a lower resolution like 640x480, but I found that the image overlays are almost unreadable.