Blackout is a convention for responding to a reverse bid

Reverse (bridge)

A reverse, in the card game contract bridge, is a bidding sequence designed to show additional strength without the need to make a jump bid; specifically two suits are bid in the reverse order to that expected by the basic bidding system. Precise methods and definitions vary with country and...

, whereby the responder can show whether or not he has any better than a minimum response. The responder bids the lower of 4th suit or 2NT to show a hand that has no game interest opposite a reverse. All other bids show reasonable hands, generally in the 8+-point range. The objective of Blackout is to allow the constructive game going hands to slow down the auction and give more room for the strong auctions that may lead to potential slams.

Responder then rebids as follows:

• A rebid of his own suit shows 5+cards in the suit
  Suit (cards)
  In playing cards, a suit is one of several categories into which the cards of a deck are divided. Most often, each card bears one of several symbols showing to which suit it belongs; the suit may alternatively or in addition be indicated by the color printed on the card...
  
   and is forcing one-round, but does not promise any extra strength.
• The cheapest bid of 4th-suit-forcing and 2NT shows exactly 4-cards in his own suit and a minimum hand for his 1-over-1.
• The non-cheapest bid of 4th-suit-forcing and 2NT shows exactly 4-cards in his own suit, forcing to game but not suitable for any higher bid.
• A preference back to the opening suit or raising the reverse-suit are both natural, game-forcing with mild slam-try or better.

Examples

Auction A

North	South
1	1
2	?

South's rebid options

Bid	Meaning
2	artificial: exactly 4-cards -suit, and minimum hand for 1
2	5+ -suit, forcing one-round, but does not promise any extra strength
2NT	artificial: exactly 4-cards -suit and is game-forcing, but hand not suitable for any higher bid
3	game-forcing with 3+ -support. At least mild slam-try
3	game-forcing with 4+ -support. At least mild slam-try
3	game-forcing with 6+ -suit
3NT	natural with stopper in 4th suit

Auction B

North	South
1	1
2	?

South's rebid options

Bid	Meaning
2	5+ -suit, forcing one-round, but does not promise any extra strength
2NT	artificial: exactly 4-cards -suit, and minimum hand for 1
3	artificial: exactly 4-cards -suit and is game-forcing, but hand not suitable for any higher bid
3	game-forcing with 3+ -support. At least mild slam-try
3	game-forcing with 4+ -support. At least mild slam-try
3	game-forcing with 6+ -suit
3NT	natural with stopper in 4th suit

Variation

An alternate scheme:

1. After a reverse, the cheaper of fourth suit and 2NT is forcing, showing a relatively weak hand. Opener bids the next bid upwards unless he has game values.
2. If a rebid of responder's suit would be cheaper than fourth suit then it is non-forcing.
3. Other bids are natural and game forcing.
4. Responder's 3NT rebid after bidding the cheaper of fourth suit and 2NT is a mild non-forcing slam try in opener's first suit.

Deficiencies

The precise scenario wherein Blackout is invoked occur quite rarely. Playing one session of bridge

Bridge

A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...

a week it is likely that Blackout would be used no more than once or twice a year. The conventional meanings attached to various bids, above, are likely to be forgotten.

The following is a critique of Blackout:

There are a number of conventional treatments and Blackout is the best known. 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. There are several variations on this theme, reverse Blackout for example. These are my criticisms.... 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. The opener has begun to describe his hand, in fact he has, generally, placed nine or more of his cards in two suits. 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. Using 2NT as the prime means of continuing with strong hands is simple, and it frees up other bids, even in the fourth suit, as natural and limited. Mis-fit hands can be diagnosed quickly and the level kept low. To use a new suit bid for no other purpose than to announce a bad hand is self-evidently wasteful. 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.