Blackwell, who has seven months remaining on his Leeds contract, was Sheffield United's head coach for seven seasons before he moved to Elland Road in 2003 but is still a firm favourite with Blades supporters.
However, Blackwell is unlikely to be tempted back to South Yorkshire, he has previously stated his ambition is to manage Leeds for the long-term, but that is unlikely to stop his old club making an approach.
The Blades, currently second in The Championship, are deciding whether to accept Portsmouth's approach. "We are currently considering an approach that Portsmouth have made for our manager," their vice-chairman Terry Robinson said. "Discussions are on-going on how we will respond to that approach and at this stage there is no more I can say."
Pompey also released a statement confirming their interest in Warnock.