The heroic opportunity window is in MainHUD.Heroic, but I didn't see any effector in there for the flashing. Looks like it's a case of the game doing it manually.
In UIBuilder, the effector you probably want is called CycleOpacity and isn't too difficult to get working.
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'Tetht the printhiple, tetht the printhiple,' muttered Igor. 'Thorry, thur, but Igorth do not "tetht the printhiple". Thtrap it to the bench and put a good thick bolt of lightning through it, thatth our motto. Thatth how you tetht thomething.'
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