If one is carrying a blade for self defence the first time your attacker should become aware if it is when he suddenly looses use of a limb or limb functions. His next clue should be the bleeding.

I have some CCTV of a close friend of mine who thought a guy was throwing punches at him. He was fending off each one and in the end used a garbage bin on his attacker as the guy kept pressing the attack. After it was all over I had to point out to him that the other guy was actually trying to stab him and he had not seen the blade at all.... !!!!!