Mr Blackman added: “Having worked with Susan, I know just how dedicated she is to our suburbs.
“I saw first-hand how she fought for Harrow residents as council leader, cleaning up the mess of the then previous Labour administration.
“I know she will bring that energy and grit to City Hall as Mayor when she beats Sadiq Khan too.
“That is why I’m supporting Susan and urge Conservatives across London to do the same.”
Ms Hall has also been backed by the Tory leader of Harrow Council, Paul Osborn, who described her as the candidate the Labour incumbent “fears”.
Mr Osborn said: “As the only Conservative Leader in London to gain control of a Council, I believe that Susan is the candidate Khan fears.
“She is the one who can win in May and will make an amazing Mayor of London.”
Ms Hall has promised to torpedo Mr Khan’s ULEZ expansion to outer London with “no ifs, no buts” if her bid to take his job is successful.
She has also vowed to take a tough stance on crime and increase investment in the Met Police, as well as building “the sort of homes people want” in the capital.
Ms Hall said: “My campaign will bring together grassroots members, councillors, MPs, ministers, and Londoners outside the party behind a set of common sense policies and show Sadiq Khan the strength we will bring to the election.
“In the coming weeks, I will set out my vision for how we beat Sadiq Khan and make London safer.”
Ms Hall is among a number of names who have entered the race to become the Conservative London mayoral candidate.
The party will name its chosen candidate on July 19 giving them 10 months to campaign ahead of Londoners taking to the polls in May 2024.