The justification for this improvement to Blackwood is simple:
The King of trumps is just as important as any Ace.

So rather than having 4NT ask for Aces, in RKCB it asks for 'Key-Cards'.
There are 5 Key-Cards, the 4 Aces and the King of the agreed trump suit.
The responses to 4NT are as follows for the so-called 1430 version:
  • 5♣ : 1 or 4 Keycards
  • 5 : 3 or 0 Keycards
  • 5 : 2 Keycards, no Queen of trumps
  • 5♠ : 2 Keycards, and the Queen of trumps
The 1430 version is the most popular, but some prefer the 3014 version which just reverses the meanings of the 5♣ and 5 Responses.

Since the trump King is a Key-Card, and the trump Queen is involved in some responses, you must obviously know what suit is trump when you use this convention. Usually this is not a problem, but if the bidding has been such that there is no clear agreement on trump, then the last naturally bid suit prior to 4NT is considered trump for the responses.

If the partnership holds all 5 Key-Cards then a 5NT bid asks for Kings. Here you must remember that the King of trumps has already been considered as a Key-Card so it is not counted as a King. The responses are the same as for regular Blackwood:
  • 6♣ : 0 Kings
  • 6 : 1 King
  • 6 : 2 Kings
  • 6♠ : 3 Kings
Lesson available.