I hate to be the bearer of bad news but you can not catch a Pokemon that already knows that as its an TM for soul~silver and a HM for the original silver.
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The place where it flashes is where you can find the Pokemon It would flash blue if it was found in the water
In Soul Silver, you can find Flash (TM70) in the Sprout Tower and in the Goldenrod City for the price of 1,000
Find an Abra to give flash in the cave in dewford.
Non. Flash is a HTM that you gotta find in the game!
You can buy TM70 Flash at the Veilstone Department Store.