If you dreamed about suddenly went dark, then you risk to forget something important at a crucial moment.
If dreamed you that around suddenly darkened, you risk to forget something very important at the responsible moment.
To dream that there is a blackout, indicates your lack of insight and perspective on a situation. You may be feeling frustrated or experiencing failure in some work that you are attempting. The blackout may also symbolize ignorance, the unconscious, evil, death, or fear of the unknown.
If you had a dream that suddenly darkened around, then you risk to forget something very important at a crucial moment.