The 24-year-old will have a scan on Tuesday which is expected to confirm he has cruciate knee ligament damage.
Blackstock came on as a substitute in Saturday's 2-0 victory at Cardiff, but was injured moments after scoring Forest's second goal.
"I am very disappointed for the lad, the whole club wishes him well," boss Billy Davies told BBC Radio Nottingham.
"The fans and everybody wish him well and it's a very big blow for us."
Despite being reluctant to use the loan market earlier this season, Davies said he may have to change that policy in the light of Blackstock's injury.
"We will have a look at it of course and see what's available and if the right one is there we will certainly try and get something done," he said.
"We don't want to bring players in who are not the right type, they have got to do the right thing."
Source: BBC Sport