Providing he passes his medical Carlisle will sign for The Whites on July 1 when his QPR contract officially runs out.
Blackwell, whoes first signing was Danny Pugh from Manchester United, says the 24-year-old will have a big role to play in his rebuilding plans.
The boss said: "Clarke is hungry to play for Leeds United. He's got great tenacity and he has a desire to play for the club. He's done very well at QPR and this is a chance for him to build on what he has achieved down there. Clarke is a big strong lad who is willing to learn and I'd like to think we can develop him a bit more."
QPR were keen to re-sign the former England U-21 international, but he turned down a lucrative offer in favour of a switch north.
Sheffield United were also interested, but once Leeds made their move he had no doubts. Carlisle met Blackwell for talks and wasted little time in heading to Thorp Arch to conclude matters.
Blackwell added: "As soon as Clarke knew of our interest he jumped straight on a plane from his holiday in Tenerife to come and talk to us. My understanding is that he has turned down better offers from elsewhere so wages weren't an issue. He is here because he wants to play for Leeds United.
"He will give us more competition for places at the back and there will be plenty of games next season when we need a big powerful player. He showed good character in coming back from a career-threatening injury to play a big part for QPR last season. He's an articulate lad who wants to do well.
"I haven't given Clarke any guarantees, because all players will have to earn the right to wear the shirt, but he's got a real desire to get stuck in and I'm looking forward to working with him."