The Split Screen Button drop down menu has: Fx Bypassed; Clipboard; Select Left Half: Select Right Half; Select All Which one should I choose? Thanks!

I was suggesting you check you hadn't switched it on by mistake as normally it is Off (i.e. not highlighted blue).

As for settings I use Fx Bypassed - Select Left Half. You then see half the screen with FX and half without. Useful for seeing what effect your FX are having on your original image.

Possibly not applicable to your current problem but it's a possible cause.

Just as an aside, it is possible to achieve black bar fill on MSP 16 (and possibly earlier versions) by copying the relevant event and pasting it on a video track above. Then go back to the original event and using "pan/crop" make "maintain aspect ratio" = no. This will make the event fill the screen. Then add the Gaussian Blur effect to the event and there you are. You can also vary the amount of blur as you prefer, and you can also use different effects instead of Gaussian Blur to give a variety of effects to fill the black bars. Some are very impressive rather than just being blurred.

Eric: As it happens, I cannot seem to turn off the Split Screen button (it's always highlighted blue, even when I start a blank project). Do I need to change something in my Settings (or something similar)? Thanks!

If just clicking it does not turn the Split Screen View button off (it should), you may need to reset the software. Full instructions here.

Resetting the software removes the Blue Highlight on the Split Screen Button, but I still cannot get the blur effect on the black bars in my preview window. Please advise. Thanks!