Just as the title says. A friend of mine is having this issue with his laptop. Is this normal? Can it be fixed such that there is no black screen flash?
It sounds like the transition from AC to battery power produces a glitch (for want of a better word) which the video hardware is reacting to by changing mode or something similar.
You don't say if there's any effect on programs running while the AC plug is inserted or removed.
If there is, the glitch may be caused by a power supply spike, which wouldn't be surprising.
I wouldn't describe it as normal behavour. Perhaps a filter capacitor in the power supply needs replacing...
This happens because unplugging the power cord changes the screen's refresh rate from 60Hz to 50Hz, and vice versa. The power manager is supposed to control for this, but sometimes it can't.
According to a thread in the official Lenovo forum, the solution is to upgrade to the latest display driver.