A blade is any sharpened edge made with the intention of cutting things (but we guess you already knew that).
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.