I would wonder at the reasoning. A knife, in and of itself, as all are aware, is not good or evil. It can only do what the wielder intends. I can see where one might want to garner a special protection over oneself, but at this point, why not ask for a blessing on yourself? Maybe, as a protection against accidental breakage, or supernatural sanitization, it would have some merit. Of course, I'm no expert, and there may be some special atributes granted by such a blessing. Doesn't hurt to have it done.
I, myself have a large "coin" that has been blessed by the Pope (John Paul II), that I received from my aunt, who happens to have been a Nun, when she received it in Rome. It has no special power, that I'm aware of, but if one has faith, it may be considered "lucky," I guess.
The power of faith shouldn't be laughed at. So, to each his own. I myself believe in the power of prayer. Maybe not over my mundane possessions, but I have had my family and pastor pray over my house, so why not?