Bolt, who was beaten by Blake in both the 100 and 200 at this year's Jamaican trials, set his world record of 9.58 seconds at the 2009 Berlin world championships. He also reduced his own world 200 record to 19.19 in Berlin.
"If he gets a Blake start, we are looking at 8.9, 8.88, three eights in the 100 metres," Smith told Reuters on Tuesday.
"If he gets a Blake start in the 200 and comes off the turn with that technique of his, he might really go sub-19."
Smith set a world record of 19.83 seconds in the thin air of Mexico City at the 1968 Olympics, a mark which stood for 11 years. On Smith's own reckoning if he had not lost speed by raising his arms in triumph over the final strides he would "have been pushing 18.9, 19.1, 19.2."
"I thought I was great when I ran," Smith added. "I didn't have the start and I didn't have the power.
"My average speed was quite close to Usain Bolt's speed. But he gained his full speed I would say five metres, six or seven strides sooner than I did. He's so powerful that he can get full speed 40 metres out in the 100, 120 metres of the 200.
"He's taller than me and more powerful. It scares you."
Americans Smith and John Carlos, who finished third in Mexico City, provided one of the iconic images of the turbulent 1960s when they bowed their heads and raised black-gloved fists at the victory ceremony in an unprecedented protest on behalf of their oppressed black compatriots.
Smith was in London for a brief visit at the invitation of the campaign group Operation Black Vote, who paid for his fare, where he will help promote the film "Salute", which tells the story of Australian Peter Norman who finished second in Mexico City. Smith will also attend the athletics competition at the London Olympics next month.
Norman, who died in 2006, wore a human rights badge on the podium in support of Smith and Carlos. None of the trio was ever selected to run for his country again.