Blackburn had already gone close through David Bentley and Christopher Samba had hit a post before Ronaldo took his goal tally for the season to ten and with it United's seventh successive Premier League home win.
United are now top of the Premier League, three points clear of Arsenal who have a game in hand.
"They are a good side, no question about that," Ferguson told Sky Sports.
"In the first half, in particular, it was an end-to-end game.
"The decisive moment was the header and then a quick goal after it killed them really and then we had easy possession of the ball.
"I thought the lad (Dunn) was a bit unlucky. He tried to read Saha and thought he was coming back towards goal and caught him.
"The second half was a professional performance and we kept possession well. We didn't take a great deal of risk, but we could have scored a couple more.
"I'm really happy with that result because they are a good side."