The 27-year-old has now returned to parent club Forest to have surgery on a knee injury that will keep him out for the remainder of the current campaign.
"He has got to have an operation on his knee which is obviously disappointing for him, but also for us," Leeds assistant manager Nigel Gibbs told the club website.
"He fitted in really well and you can see from his performances how much he brought to the team. We are sad to see him go back."
Blackstock, who was due to remain at Elland Road until January, scored once for Leeds on his debut against Huddersfield at the end of October.