Blank Key

Blank Key is used to copy other keys that lock/unlock containers or doors around Celador. You can add them to your key ring that is attached to your character window.

Source
Blank Keys can be bought for from Tinker shops in the following towns:


 * Valus
 * Eldeir Village
 * Pyros Landing
 * Oasis
 * Black Forest outpost
 * Helm