5' overhangs (3' recessed ends) can be filled in, but 3' overhangs will not be removed because the enzyme lacks an exonuclease activity. DNA Polymerase I, Large (Klenow) Fragment (NEB# M0210), T4 DNA Polymerase (NEB# M0203) and the Quick Blunting™ Kit (NEB #E1201) are the best choices for creating blunt DNA products.