DNA is blunted using T4 DNA Polymerase (NEB #M0203) which has both 3 → 5 exonuclease activity and 5 → 3 polymerase activity. T4 Polynucleotide Kinase (NEB #M0201) is included in the enzyme mix for phosphorylation of the 5 ends of blunt-ended DNA for subsequent ligation into a cloning vector. This kit is optimized for blunting up to 5 g of DNA in a single reaction. 
Blunt Enzyme Mix supplied in: 100 mM KCl, 10 mM Tris-HCl(pH 7.4), 0.1 mM EDTA, 1 mM dithiothreitol, 0.1% Triton X-100 and 50% Glycerol.