Under normal circumstances, when AC adapter is off a thinkPad (it means on battery only), the machine's display will dim slightly according to the power plan set. Conversely, the display will bright up again when the machine hooks up to power with AC adapter.
However, my T420 screen blacks out and then turns back on when removing main AC power and re-attaching it. Note that this never happens once to either T61 or T41p.
Initially I assumed it was because I had an external monitor attached, however, it happened even without the external monitor. Then I tried checking for video driver updates but nothing is mentioned in the driver note thus I never do the update.
1) Check Power Manager:
If remove AC adapter, does the refresh rate change to 50hz when unplugged?
If AC is plugged in, does the refresh rate changes back to 60hz again?
2) Check the Intel Media profile at the power settings.
If the 'On battery' is "Maximum Saving" mode then it should be swapping from 50Hz to 60Hz.
Change the power settings to 'Balance' or 'Maximum Performance' and see if it's still changing from 50Hz to 60Hz.
If all else fails, try updating the power manager and video driver as it could be a bug from previous version of driver.