After disconnecting my battery I had no message at all about the key card.
It won't work with someone else's card as their card is not associated to your head unit.
Your current key card should still work!
1) Turn the unit off
2) Remove your card
3) Turn the unit on
4) Enter your key card into the unit
5) Message "KEYCARD OK" should flash up
If this is still unsuccessful then you'll need the Master Code for your head unit.
If you can't find it with your car's documents then I'm sure Blaupunkt (maybe even SEAT) can issue you with a new one!
Here's the process to do it:
1) Switch radio on - Display shows "KEYCARD NOT OK"
2) Turn it off.
3) Press and hold "BND.TS" & "1.MIX" and then...
4) Switch radio on - Display shows "0000 MASTER CODE"
5) To change the code press Tuning Button "^" until the correct digit is shown
6) When the correct number is shown press Tuning Button ">" to enter it and move to the next one.
7) Repeat this for all 4 digits.
8) When the correct code number is now shown press "MENU.OK" to enter it.
9) The new key card is accepted if the unit returns to playback mode and the message "KEYCARD OK" flashes up.
EDIT: Using the Master Code will work with your current key card, even if that card does not currently work.