Kosher doesn't mean blessed by a rabbi. Like people of many faiths, Jews traditionally recite a blessing before eating, acknowledging the sacred source of all things, and the sacredness of acts such as eating. But that’s not what makes food kosher.
Kosher means fit for use, according to Jewish tradition. In the case of food, it means eating according to a biblically-rooted code which asks that people eat with reverence for all life, and nurture the awareness that there is a connection between what we put in our mouths and how we act in the world.