The objective of Blackout is to allow the constructive game-going hands to keep the bidding level low and give more room for the strong auctions that may lead to slams.
In its simplest form, when responder’s second bid is the cheaper of 2NT or the fourth suit it is “Blackout” advising partner that the bidder is minimum, all other continuations are game forcing.
So much more commonly will the partnership be at the limit of its resources, simple bids like a return to opener’s first suit or a repeat of responder’s are better played as natural and non-forcing.
A strong responder should be able, through 2NT, to allow opener to continue to describe, to find the perfect fit whenever it exists, search out the best game or make a slam investigation.
Having at your disposal, means of getting out cheaply and accurately, permits opener to reverse, and thereby not conceal his actual shape, on any hand above the minimum re-bid range.