The Prime Minister came to City Hall on Tuesday afternoon to sign the book of condolence for the victims of Tuesday's explosions opened by the Mayor of London on Monday.
"With deepest condolence for all those who lost their lives and for the families who mourned and with heartfelt admiration for London, the greatest capital city in the world," Mr Blair wrote.
On Monday afternoon the Princess Royal had also made a surprise visit to the home of the Greater London Authority to sign the book. She simply signed: "Anne".
Entries for the book of condolence can also be submitted online.
The book was opened at a special ceremony on Monday morning led by the Mayor of London Ken Livingstone and attended by Culture Secretary Tessa Jowell and Metropolitan Police commissioner Sir Ian Blair, as well as religious leaders.
Ms Jowell wrote: "The strength of London and her people is in diversity and tolerance. Our deepest sympathy is with the grieving families."
On Friday, on his return from the G8 summit in Gleneagles, the Prime Minister paid a visit to London Ambulance Service HQ in Waterloo Road where he congratulated staff on their response to the bombings.
• A two minute silence will be held on Thursday at 12 noon. It will be observed at Southwark Cathedral where the silence will be followed by prayers led by Canon Bruce Saunders.