Making a box appear and disappear

Hi guys 

I am a little stumped by this one. Wonder if you can help?

I have a video on a slide. 
The video is set up so learners can fast forward and rewind as they wish. This is essential.

The video I have been supplied with was about 30 mins long. So I have cut out the 85 secs that I need. The video has subtitles hardwired into it. No one has the original any more.

At 85 seconds, the person speaking in the video stops talking, and it cuts to a different scene where she continues, but the subtitles show her final words AND the initial words of the next scene. 

I can't delete the subtitles. 

So I tried a variable to say when the timeline reaches 83 secs, show a black box that covers the pointless subtitles. But as the learner has full control of the video, they can fast forward and "beat" the timeline. So the box never shows up. If they DO just sit there and watch the whole thing, it works. But then if they rewind, the box remains.

Does anyone have a fancy way to have the following:
Show black box when media reaches 83 secs. Hide black box when media is between 0 and 83secs.
Or
Show layer X when media reaches 83 secs, hide layer X at all other media time points.

Thanks!!