Shearer, 39, is in the hunt for another manager's job after his stint in the Toon hot-seat last season.
But Birch, 51, made it clear that Blackwell is staying put and will NOT be replaced by the St.James' Park legend.
"We have worked together before, and I know him to be an extremely hard worker and well organised.
"I think he was extremely unlucky at Leeds, and also here with injuries this season.
"He has not got the tools he needs in terms of player fitness, but I am sure he will sort things out."