A lot of times, rather than using "merge clips", you can take a full bin of media, select all, and "create multicam sequence" as a batch. If you've a bunch of clips and accompanying audio of different events so no camera/audio bits overlap any other, it will make all of them into single-sequence "clips", and you can choose which audio to keep and if you wish to dump other. You have now joined media, without some of the hassles & limitations of merged clips.
Sorry for the late reply! And thank you for your response, R. Neil Haugen. Yep, I'd come across that alternative workflow when searching online for a solution... unfortunately it doesn't take care of what was happening in my instance.
Having exhausted all other possibilities, I gave up on merging that clip and moved on to the next... and that one worked properly. I now wonder whether there's something corrupted in that problem clip, that results in the black screen... hard to imagine, though, as the clip so far behaves and plays back just as any other good camera clip would, other than that sync'ing problem.
Merging subsequent clips, I have noticed a few of them appeared black at first... but then the image appeared; there was a time lag. (I'd certainly given that other one plenty of time, though.) Anyone experience anything similar...?
Huh. No, actually the clips I've merged and multi-camed together both just happened in a few seconds.