1) Ensure that "Quantize to Frames" is enabled
2) Try trimming one frame off the end of the first event and/or one frame off the beginning of the second event and then manually rebutting the events. I've noticed the issue you report occurring when the frame rate of the source is not exactly 29.97 such as with the iPhone (variable frame rate) or some cameras that record at 30.00 fps. Also, you might want to "disable resample" on these events before rendering to prevent ghosting.
Place the cursor at the gap where the video events change Clip-1 to Clip-2, then Bottom Right use the + to expand the timeline - is there a gap? If so that's why it's black.
Can't say I ever had that as an issue, but then I push them together, and will usually overlap to achieve a smooth transition. Otherwise it's seen as a "jump-cut" - not that there's any reason not to, - you need overlap if you want to use any of the many Transition presets provided. They will occur during the overlap time-period. Hope that helps.
Let us know when it's sorted. . . .
This site uses a naughty-words eliminator - so when I butt two pieces of film together it won't let you read what's written . . . .Oooer. so I have substituted "push" however I mean with two t's but. OK?