Although the series has now ended, a POI fan recently tipped us off that Reese's knife of choice, throughout the series, has been a Kershaw Tanto Blur.
CBS's Person of Interest is a science fiction crime drama that just completed it's fifth and final season. One of the show's heroes is John Reese (actor Jim Caviezel), a former Special Forces operator and CIA black ops officer. Turns out, Reese's knife is a Kershaw Blur. You can see it clearly in this (bad—sorry!) shot of the knife as it is about to be used to perform some life-saving surgery on Sameen Shaw (actor Sarah Shahi), another of the show's main characters and a former ISA assassin.