Blackstock, 27, a peripheral figure for Forest this season despite signing a new four-and-a-half year contract at the City Ground in January, has joined Leeds for an initial three months.
"I've known Dexter for a long time, being brought up in Oxford," Leeds boss Brian McDermott told the club's official website.
"I'm really pleased we've managed to secure his services.
"He's a really good age for a striker and I'm sure he has many good years in front of him.
"We are all really looking forward to working with him."
Blackstock, who has scored 83 career goals in 306 appearances, has also had spells at Southampton and QPR.
He is expected to go straight into McDermott's squad for Saturday's West Yorkshire derby clash at Huddersfield.