I'm transferring VHS-C home videos to iMovie 11 so we can burn them to DVDs. During import the video blanks showing only black for just a second and then it goes back to normal. It happens every few seconds throughout the whole movie. I tested the tape and the VCR on a standard TV and the black flash doesn't occur.
At a guess, Id say that this is happening because you're demanding too much from you computer, while importing, and playing. Your computer may not have enough memory or processing power to both display a clear video and import it at the same time (both playing and saving the video from the VCR).
I'd only be concerned if after you disconect the VCR or pull out the tape, you still have this problem when trying to play the video.