You should have both mp4 and webm versions of your video files within your movies folder, though typically if you're missing the webm version you would just end up having no videos playing on your title screen, what kind of black screen are you getting? Like it never even reaches the title screen? Or is the background behind the title screen black?
If it's just a black screen before the title screen and you never get to the title screen then it might be worthwhile to test disabling other plugins to see if any of them are conflicting somehow.
Also ensure that your videos are converted properly from mp4 to webm, incorrect conversion can cause issues as well.
If we can find an incompatibility I'll get to work finding a fix for it.