Bolt won the blue riband 100m from Jamaican countryman Blake last weekend, but the latter's great power means he is fancied by some people for the 200.
Bolt has always maintained that the longer sprint is his favoured race - and he looked dominant in easing to his semi-final victory on Wednesday.
However Blake also qualified with ease.
"It's going to be an amazing race," Devonish, whose own chances of glory at these home Olympics were ruined by an injury in the lead-up to the Games. "I was there for the 100m and you could have heard a pin - or a bottle! - drop.
"My money is on Bolt: he says he isn't 100 per cent but after Blake ran so well before the Olympics he had time to step away from everything and focus on the task at hand.
"He runs a reasonably hard 60 metres and gets his momentum going, which brings him home. He seemed to stroll down the back straight but his time was my PB - and, in putting down 20.1s, he is no slouch - he is moving! At most championships that would medal.
"That is part and parcel of the 200m - in the 100 you accelerate as fast as you can all the way through, but in the 200 you can't hit it that hard. You can't physically maintain that throughout the race - if you try the wheels fall off.
"Usain's top-end speed is that much faster than everyone else's. He accelerates slower but he is more efficient in the deceleration phase. He uses his height as an advantage. His stride length is longer than the others': he takes 41 strides for the 100m compared with the 45 of other sprinters.
"He has a slower start because of his height, so it's his job to get into the mix for the first 40 metres, but the taller athletes then always come through. After accelerating away, it is then his job to maintain that speed."
The warmer weather could lead to some extremely fast times in the Olympic Stadium, although Devonish does not expect a world record mark.
Bolt ran the second-fastest time ever in the 100 - 9.63 - but was some way off his WR of 9.58.
"I don't think he will run a WR time [in the 200]," he added. "But the problem in the 100 was that the weather was cold, which meant Usain was never going to threaten his world record [whereas tonight he could].
"Sprinters love the heat, and it's a lot warmer today. It could be five, six, seven degrees warmer tonight than it was for the 100."
Devonish says Jamaica could even be celebrating three medals come the end of one of the most eagerly-anticipated races for years, with American Wallace Spearmon and France's Christophe Lemaitre the main threats to that.
"It could be a one-two-three for the Jamaicans with Warren Weir third, although Wallace Spearmon always snatches a medal and will do so tonight if he is ready," he said. "It's going to be close between Spearmon, Christophe Lemaitre and Weir for bronze."
Britain's Christian Malcolm, who Devonish knows well, just missed out on the final.
"Christian Malcolm will be disappointed. I train with him and he knows he can do 20.2s but he didn't run the semi-final he wanted," added Devonish.
"He reached finals at the last two Olympic Games - anyone who can manage reaching a final in these events, hats off to them."
"He now has to let it go [as he prepares to go] for the relay [medals]."
Marlon Devonish was speaking at the Mizuno Performance Centre - open to the public between the 24th July and 12th August at Centre Point Building, 101- 103 New Oxford Street. Visit www.mizunoseiei.com for further details.