In light of recent measurement of nitrogen isotope ratios in CN and HCN in several comets, and the correlation between 15N excess and the presence of nitrile (-CN) functional groups in meteoritic samples, we have reassessed the potential of interstellar chemistry to directly fractionate nitriles. We focus in particular on the 15N chemistry in selective depletion cores where O-bearing molecules are depleted yet N- and C-bearing species remain in the gas, as revealed by the recent detection of CN in dense CO-depleted cores. We show that large HC15N/HC14N ratios can be generated if the reaction of CN with N has a barrier, and suggest that cometary HCN and CN may trace material originally formed in dense interstellar clouds.