The Glasgow flanker comes into Scott Johnson's side to replace Alasdair Strokosch in the Murrayfield clash with Italy.
And Blair, who won 85 caps for Scotland including 14 as captain, believes Harley can have a major impact against the Azzurri.
"Rob is an absolute menace," said Blair.
"Even in training he was a guy you hated playing against - he would always make a real fight of the breakdown and you would never get clean ball.
"I can see him bringing that spirit with him and that being a big plus for Scotland."
Harley made a try-scoring debut against Samoa in the summer but Saturday's encounter will be his first international start.
Italy rocked the Championship with an opening victory over favourites France last week and Blair admits they pose an enormous challenge.
"I didn't think France played too badly - Italy played extremely well," said Blair.
"Normally, Scotland at home in this fixture would start as firm favourites but the tables have been turned a bit with Italy ranked nine in the world and Scotland at 12.
"It's a massive game. Pressure is a funny thing. It can work two ways; it can build a spirit in the camp and unite everyone or it can pull a team apart.
"Knowing this bunch of players, I think it's going to bring them together."
Scotland crossed the whitewash twice in their Calcutta Cup defeat to England and Blair insists there is plenty to be positive about.
"There were enough exciting things against England for Scotland to go into this game with a bit of confidence," added the scrum-half, who made three appearances for the Lions in 2009.
"Playing England at Twickenham is an incredibly hard match to open the championship with, having not won there in 30 years.
"It was a big test but there were positives to take - especially the back three [Hogg, Maitland and Tim Visser]."