Bonner is just flat out a better defender than Blair.
Blair is short (people shoot over him), doesn't play good position defense (so people go around him), doesn't rotate well (doesn't effectively help other players), etc.
Bonner, for all his lack of ability, boxes out, plays solid post D, and makes the right rotations.
And look at Blair's rebounding rate. He has statistically gotten worse at rebounding since his rookie year (though improved on scoring average). Meaning we are integrating him into the offense more but he isn't really helping us more.