One of the oldest conventions, the Blackwood 4 no-trumps convention is used to ask for aces. It is designed to keep you out of slams when you are missing key controls, usually aces. Only use the Geber 4 bid to ask for aces over an opening 1NT, 2NT or bids like strong 2 opening with an imediate re-bid in NTs. You may need the 4 bid to show a suit etc.Do not use Blackwood with a void (missing 1 ace you do not know whether it is in the suit where you hold a void), and be particularly cautious with a small doubleton in an unbid suit.
You hold: AKQJ107 AK79 KQ9 void. The agreed trump suit is s If you ask for aces and get 1 ace. Do you bid 7 and find out it is the A that's missing, or bid 6 and find that 7 is on, because partner has the A! Use cue bids instead.
So here are the options
1. Simple Blackwood 4NT asking bid
0 aces= 5, 1 ace = 5, 2 aces = 5, 3 aces = 5, and all 4 aces = 5NT
You can ask for Kings by bidding 5NT after partner's response, but should only do this if the partnership has all 4 aces! The responses are as for aces but at the 6 level
There have been several more sophisticated attempts to improve this convention and one that is popular amongst tournament players is Roman Key Card Blackwood (RKCB)
Once a trump suit has been agreed then the King of that suit is regarded as a "key" card , like an ace. The agreed suit can be directly agreed or by implication. If NO suit is obviously agreed then the LAST suit bid is the key card suit. So there 4 aces and the king of trumps available in responses (sometimes known as 5 ace Blackwood)
2.The responses to RKCB are:
5 = 1 or 4 key cards; 5 = 0 or 3; 5 = 2 without the Queen of trumps; 5 = 2 with the Queen of trumps. If after you get a response of 5 and you are sure it shows 4 controls you can enquire about the Q of the agreed trump suit. However if you are uncertain about the number of key cards held when partner responds 5/, then you sign off in the agreed trump suit. Partner with 4 or 3 will advance the bidding further. (It is usually easy to distinguish between the holdings for a 5 (1 or 4) or a 5 (0 or 3) responses from the bidding. You rarely ever ask for aces holding 0 or 1 ace.)
The main disadvantage is that if you are missing a key card it could be an ace and leaving you with 2 quick losers.
Remember that some players reverse the 5/5 responses
You can ask for Kings by then bidding 5NT in the normal way as for standard Blackwood
The advantage of using 1430 responses are that you can also then find out about the valuable Q of the agreed trump suit.This is done by bidding 5, over a 5 with responder signing off in the agreed trump suit, with no Q of trumps. You need to agree what you will do if you hold the Q and extra values.