A blade is any sharpened edge made with the intention of cutting things (but we guess you already knew that).
Live steel is strictly forbidden on the field in the the rules of the list. For heavy fighters in the SCA the "blades" on weapons used in combat are in fact strips of electrical tape along the side of the rattan to indicate the cutting edge. The weapon is "blunt" to limit excepionally serious damage being inflicted onto fighters (although being hit hard enough with what is essentially a club can hurt anyway).
If you are going to be using a knife or similar tool in a way that could hurt anyone you should make sure that no one is nearby and warn anyone who is (this seems obvious but accidents do happen). Some people in the SCA like to say "clear" when they unsheath blades near others.
The only group of fighters, for which a metal blade is allowed, are rapier combatants, and only approved blades are allowed. They must have the prescribed amount of flex, must be unsharpened and must have the tip cut off and replaced by a rubber blunt that is securely held in place.