That's true, but Blair doesn't have lateral speed, can't even jump much, nor the wingspan of Darko, so he cant anchor the defense. The Spurs have to play with someone who anchors the defense when Diaw, Bonner, Blair are on the floor, or when they use Leonard or Jack at 4. The problem with Blair is that he doesn't have midrange game, so he can't be used with Splitter. And since he can't defend, he can't be used with Bonner.
With 3 shotblockers, the Spurs also can put 2 centers against certain teams, like the Lakers, and still have someone to anchor the defense when the bench kicks in.