IN POKEMON WHITE! I do not have black so I am only answering for white. Go to Castelia City. if you flew, walk two streets left or if you walked from the east walk 3 streets lefts or if you walked from the north, go to the second street from the left (this street is called narrow street). start walking down and when you get to the big green dumpsters a guy will pop out and give you the TM 70 Flash.
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get to the south entrance and go to the middle of the1st floor and give your pokemon the move flash then press "a" at the purple stone
Any Pokemon that can learn a electric move
Flash is an initial move learned by Volbeat in Generation IV. For the Pokemon who can learn Flash through TM70 in Generation IV, see http://bulbapedia.bulbagarden.net/wiki/Flash_(move)#Generation_IV_2.
Non. Flash is a HTM that you gotta find in the game!
When a Pokemon has the move Flash, you go into Party, then select the Pokemon with Flash using the 'A' button and there should be an option that says "Flash". It is only usable in dark locations or within battle.