When you have used the Keycard Blackwood convention and have discovered there is one missing keycard, that doesn't necessarily mean that a slam is on. If the Queen of Trumps is missing too, you may have two losers if you don't succeed in finding her.
In order for the slam to be a good one, you need to verify that your side has the Queen of Trumps.
When your partner has responded with 5♣ or 5♦ to Blackwood, the next bid (5♦ to 5♣ and 5♥ to 5♦ (if Hearts are not trumps)), will ask your partner for the Queen.
If he doesn't have it, he will go back to your agreed suit at the lowest level.
Any other bid will show that he has the Queen and he may show an additional King, by bidding the suit that it's in.
Let's try this out with a few examples.