Darius Henderson was shown a straight red for what was deemed to be an elbow on Forest's Paul Anderson off the ball less than a minute into the second half of the Coca-Cola Championship match.
Blackwell said at the time: "I can't say what I want to say, the game, in terms of that, is wrong.
"A manager should be able to come out and say what he wants to say, otherwise I don't see the point of the interviews.
"As managers we cannot speak our minds in press conferences because we get punished by the FA for berating the officials.
"There were a lot of decisions we didn't get but I have to be careful what I say. The whole crowed was mystified on five or six occasions following decisions...it shouldn't be me talking about the referee, it should be the media.
"The referee (Nigel Miller) has a responsibility to control the game and I think he lost it.
"It was a big game for both sides, two teams and a crowd full of passion and then there is someone in the middle who doesn't seem to understand it."