Didn't find it myself (it was a ticket on the Nexon forums) - but it worked for me:
A bug is in existence that allows a user to get a black key without payment of NX or GP. The bug goes as follows:
1. Visiting the Nexon website through
http://combatarms.nexon.net.nexcorp.tk/combatarms.nexon.net/
2. Clicking on "store" after logging in.
3. Visit the black key page.
4. The black key being spontaneously placed in the user's inventory.
... This should be fixed for people not to get black keys unless they pay for it.
Don't abuse it, you'll get it patched quickly.