Kobe Bryant has been on the NBA All-Defensive time 12 times - 9 on the 1st and 3 times on the 2nd. He's ranked #14 on the all-time steals list and #1 among active players. So it would be tough for me to tell anyone that Kobe was an underrated defensive player but I can say he was an underrated shot blocker and one of the best shot blocking guards ever. He wasn't was of those guards who only got their blocks on chase-downs either, Kobe was one of those meet you at the rim type guys.
To back up my claim, here's a couple of videos: The one above is a 13 minute mix of Kobe blocking a bunch of NBA Stars - From KD to KG to LeBron to MJ - and below is a 4 minute compilation of Kobe just blocking people - mainly with his left hand?!