United were not at their best, but they ground out another three points and they have now kept five consecutive clean sheets in the league at Bramall Lane.
Blackwell admits that staying in the top six is his main objective, but the former Leeds boss believes they are still in with a chance of finishing in the top two.
He said: "I think over the last two years we have been the form side in the last half of the season.
"I think last season we were nine points clear of everybody else over 10 games after Christmas and I think the season that I came here we had 14 games and we were four points clear of everybody over 14 games, so if we can achieve any of those two we'll have a good run.
"But right now, we've got to make sure we stay in the top six, and don't kill ourselves.
"This is a tough league, you saw Middlesbrough change themselves today and I thought they were much more physical and a much harder nut to crack than they have been of late.
"In this league, as Gordon (Strachan) was saying before the game, it's a tough, tough league because every game you can't sit down and predict the score.
"What you have to do is earn the right to play and from that point of view I was pleased with the good aspects of our game."
Blackwell revealed that Matt Kilgallon had missed the game through injury, but admitted he had no idea if the in-demand central defender will stay at Bramall Lane.
Boro could count themselves unfortunate to come away with nothing after a battling display and they could have had a penalty when Jeremie Aliadiere went down under a challenge from Chris Morgan just after Cresswell's goal.
Gordon Strachan, who handed debuts to new signings Barry Robson, Willo Flood and Chris Killen, vowed to make further changes to his squad with Boro having lost six of their last seven games in all competitions.
The former Celtic boss said: "It's a concern. Fortunately I've been about for a few years in this job, I think it wouldn't be great for a young manager in this position, but for somebody with a bit of experience, I do know what has to be done and I have to change it, it's not great fun, it's not nice, I wouldn't recommend it for anybody but you have to deal with it."
He added: "Until the goal went in, which is something we're not happy with because we did warn not to turn your back on free-kicks, but until then I don't think Danny (Coyne) has had to make a save.
"It felt reasonably comfortable, I don't think there was a real problem in the game and the goal changed it a wee bit.
"We tried to go a bit wider and make the game a bit more stretched, but it didn't work for us."