HI
I have found that this "black flash" does not always happen but it is a definite effect and it may be version specific. I am pretty sure that it is connected with the last frame of a clip possibly being left as a black blank frame in some versions. Retrimming the ends of each clip does not help, nor does closing any possible gap or pulling the ends of the clip back. The black frame can sometimes actually be seen in the sequence preview screen if one zooms in and steps through the join one frame at a time. I also had the feeling when I saw this, that the black line that goes between clips on the sequence track is slightly more pronounced (or wider?).
You can get around this unwanted effect by pulling the right hand clip up onto video track 2 and overlapping it back it a little (a frame or two) onto the preceeding clip.
Nat