OK Sorry, this would be simple in Final Cut Pro X which I have been using lately. I wrote this last night, but my browser had a glitch and I lost it. I am trying to recreate it.
In iMovie, there are a couple of ways to approach this. I don't have time right now to try them, but I will outline them and you can see if they will work for you.
1) You could select the beginning segment that you want to cover with the Title. Then right-click/Split Clip to separate this portion from the rest of the clip. Next select this newly split clip. Right-click and choose Detach Audio. Finally, drag the black background on this clip and choose Replace in the popup menu. It would be tricky to add a transition without messing up the timing of the audio.
2) You could select the beginning segment that you want to cover with the Title. Note exactly how many seconds and frames it is, and then Right-click/Delete Selection. Add the Black background and title and make them this length. Finally, select the same length from the exact same clip in the Event and drag it and drop it on the black background. In the popup menu, choose Audio Only. Once again, it would be tricky to add a transition without messing up the timing.
Your best shot at adding a transition without messing up the timing of the audio would be to try setting Transition Overlap to Half. Do this by double clicking on the transition and choosing Transition Overlap - Half in the popup.
I am not sure if this would work for you. See HELP here for more on Transition Overlap.
If you think you might be doing a lot of music videos, I highly recommend that you get Final Cut Pro X. It makes stuff like this easy, and it makes it possible to easily use multiple cameras.