Because a preempter will not have many Aces, it doesn't make sense to use Roman Key Card Black over a preempt. It is better to use Preempt Key Card Blackwood over a preempts of 2♦ through 3♠.
Preempt Key Card works as follows:
Responder's 4♣ bid is artificial, Preempt Key Card. The preempter answers using steps:
- 4♦ shows 0 Key Cards
- 4♥ shows 1 Key Card without the Queen in the preempter's suit
- 4♠ shows 1 Key Card with the Queen in the preempter's suit
- 4NT shows 2 Key Cards without the Queen in the preempter's suit
- 5♣ shows 2 Key Cards with the Queen in the preempter's suit
Over a 3♣ preempt, a 4♣ response is simply advancing the preempt. Responder bids 4♦ as Preempt Key Card. The preempter's responses are all a step higher.
For Key Card responses, the preempter includes the 4 Aces and the King in the preempter's suit.