Black frames can appear in your exported video, for example, if your project contains a loop region which extends beyond the end of the last event in your project. This can be useful if you want to fade to black at the end.
Black frames can also appear at the beginning of your movie if you leave extra space in front of the first video event. 

If you don't want black frames to appear in your exported video, make sure the loop region is only as long as the video material in your project.
Be sure to also enable the option "Render loop region only" before exporting.