Blackwell facing lengthy spell on sidelines.
The 34-year-old all-rounder has been ruled out for "six to eight months" after an operation on his shoulder - the third he has had in four years.
And it could effectively end his Durham career. He was out of favour last season, and out of contract at the end of the next.
His last competitive game for Durham was in May. After that he was told to go away and get fit, and when he did, he was loaned to Warwickshire. He won the County Championship at Edgbaston, and Durham coach Geoff Cook said this week he had been looking forward to welcoming Blackwell back into the fold until his latest setback.
The former England international's shoulder has been a constant problem while at Durham. He had an operation on it two winters ago, having had one in his Somerset days.
This year he needed an injection early in the season, and another while at Warwickshire.
With Blackwell out the picture and funds for new signings tight, there will be a huge onus on 22-year-old Scott Borthwick.
He and Gareth Breese are the only other senior spinners at Durham, and Breese tends to be seen as a one-day specialist.
Durham will have to keep their fingers crossed that Borthwick does not force his way into England's plans, and he did them no favours yesterday, taking 6-38 as the England Performance Programme - effectively the second string - beat a DY Patil Academy XI by an innings-and-193-runs.
Borthwick also chipped in with the bat, and England cruised to victory when they bowled their hosts out for 87.
Surrey fast bowler Stuart Meaker took 3-8. He was called up to the senior squad as cover for the injured Steven Finn but now that he has proved his fitness, Meaker will stay with the EPP.
SETBACK Ian Blackwell