The only possible explanation is this. The official scorer MIGHT have decided that Blalock was going to try and throw out the batter at first, instead of forcing the lead runner at second. On the play, he never fielded the ball cleanly and could not even attempt a throw. If that is the case, the runner at second would have scored on the single despite the error.
Now, the fact that it was scored "Fielders choice, reached on error" seems to me to contradict what I just wrote. It all depends on whether or not the official scorer thought that the runner would have been on 2nd base with or without the error.
Personally, I think the run should be unearned.