You do have a fair point that Blackett must have done something to get himself into the first team, but also keep in mind who Blackett has to compete with for his spot.
Blackett's only competition for the roles he can play are Shaw, Rojo, and Blind (LB and LCB).
Keane, Thorpe, and McNair have Smalling, Jones, and Evans to compete against (RCB).
We have 6 RCB's who can possibly play for the first team at RCB. That results in lots of competition for Keane, Thorpe, and McNair. On the other hand, we have 2 players who can possibly play for the first team at LCB. Thus, Blackett has fewer obstacles to overcome than Keane, Thorpe, and McNair.
Also, it says quite a lot that van Gaal would play Blind at LCB against Crystal Palace than play Blackett there. It's apparent that van Gaal doesn't see much promise in Blackett as a central defender in a 2-man-defence formation. I would even say that he doesn't see much in the 3-man-defence formation as I'm sure that, if Rojo and Blind were available, he'd play one of those two ahead of Blackett.
Regardless of how Blackett's getting his first-team appearances, it's a great chance for Blackett to assert himself into the first team and prove van Gaal that he's a worthy first-team player. However, for me, he looks out of sorts as a central defender and is still much better off as a full back, which I see as his natural role.