My gut feeling's telling me that van Gaal considers Reece to be more of a central midfielder due to his composure on the ball, decent technique, and good positional sense. Blackett doesn't really have those, but he is a good athlete and did look promising as a left back in the past. The fact that Blackett didn't play much during his loan spells might not have helped with his development as a player. However, if he knuckles down and works hard, his potential may be realised. However, I'm probably being optimistic at this point.