Blackwood is the most popular of the slam conventions. It is used to find out if
there is enough Aces and Kings to bid a slam. A bid of 4 NT signals to your
partner you are using Blackwood.
The Blackwood responder counts the number of Aces in their hand and responds according to the result:
5C : 0 or 4 aces.
5D : 1 ace.
5H : 2 aces.
5S : 3 aces.
A count of the Kings in partner's hand is requested with a subsequent bid of 5 NT. Your partner replies in a similar fashion at the 6 level:
5C : 0 or 4 kings.
5D : 1 king.
5H : 2 kings.
5S : 3 kings.
Caution: The 4 NT is considered to be Blackwood in various notrump sequences including direct raises and after Stayman. A bid of 4 NT over partner's opening 1 NT is not Blackwood, but a quantitative raise of the opening bid. In notrump situations the Gerber Convention is more appropriate.
Caution: Blackwood should only be used when the number of Aces is what you need to know. If you need to know if your partner has a specific Ace, use Ace-showing methods of slam bidding.