The 70-year-old boss admitted it had been a 'long night' in the dugout but the defending champions eventually got the breakthrough to go five points clear at the top of the table
United dominated the match at Ewood Park but did not find the breakthrough until the 81st minute when Valencia drilled home a cross-cum-shot, before Ashley Young secured a 2-0 victory with a precise finish.
"Coming so late in the game, it is an important goal," Ferguson told Sky Sports.
"It typifies the history of the club. If you don't try to shoot, you're never going to find out if you can score like that.
"There was amazing power on it. It could have been a cross, could have been a shot, but it doesn't matter. There was great power [on the attempt]."
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Sir Alex felt that for all of the possession his team had enjoyed, they had struggled to really test Blackburn goalkeeper Paul Robinson.
"It was a long night," he admitted. "We just had to persevere and we got our rewards for it.
"We weren't really creating any clear-cut chances – Wayne [Rooney] had a good free-kick saved by the goalkeeper, and there was a lot of great crosses by Antonio that weren't finished off.
"We always had them under the cosh, but we had to be careful of their counter-attack. They were dangerous at that point."
Steve Kean's team almost took the lead on the stroke of half-time when David de Gea denied both Marcus Olsson and Grant Hanley with excellent saves.
De Gea had struggled when Blackburn recorded a shock 3-2 win at Old Trafford in December.
"The progress [of De Gea] is obvious, you can see that," Sir Alex enthused.
"The boy is growing in stature and confidence and he's always had great ability. He made three great first half saves and that's the kind of threat Blackburn offered to us.
"They presented us with this counter-attack, they try long balls and they're dangerous with that, they're very dangerous. David has kept us in the game.
"The first half we hit the post, we had a penalty claim that I don't think was [a penalty] and a lot of possession as you saw. Just to get that goal was very difficult with the way that Blackburn defended so deeply."
The 70-year-old manager was not sure that his switch to 4-3-3 had helped United but hailed the club's supporters for playing their part in the Red Devils' important win after around 7,000 of them made the short journey to Ewood Park.
"It [the change in formation] gave us a stability and a security in midfield but it didn't necessarily give us the threat we were looking for," Sir Alex explained.
"Ryan Giggs coming on made a difference and certainly Ashley Young coming on made a big, big difference.
"The fans were fantastic, they were unbelievable. They deserve the result tonight because they never stopped, they urged us on the whole way.
"They almost sucked that ball in [to the goal]. It's frustrating at the end of matches sometimes and that's where it was right tonight."
The result puts United in control as they look to defend their Premier League title, giving them the opportunity to, at least briefly, go eight points clear at the top when they play QPR on Sunday.
"We've scored more goals, which is a big step for us – the goal difference deficit has gone down to only one goal," Sir Alex added.
"It could come to goal difference, who knows. It's significant that we've now scored more goals than Manchester City. We look as if we can score.
"We've still got seven games left and my experience of these situations is that it doesn't matter what the points total is at the moment – what is really important is trying to win Sunday's game."