In HitFilm 4 Express, if you accidentally put two full Linear Wipe Transitions, you get the effect of a black screen being wiped between the two clips. I ended up watching several videos and basically with the help of someone else I was posting back and forth with on a Facebook group pretty much troubleshooted the problem without even trying. The problem occurred this morning when I uploaded one of my now two weekly videos, after I was watching it to make sure it uploaded without problems I noticed messed up transitions where a black screen would appear between two clips instead of a nice wipe across the screen. I know this seems very easy to find, but I was looking through my timeline frantically for a long time and until I reached the right zoom level, didn't notice the transition icon (I am almost brand new to HitFilm, I only recently dumped Windows Movie Maker entirely) So the parable is: when bridging two sources, don't overlook the middle-man! I hope this helps someone, I know it was nagging at me until I off-the-wall came to the conclusion.