Ben Youssef Metie, 29, of the Ivory Coast, notched the morning's second-fastest time of 10.03, edging USA hopeful Trayvon Bromell's 10.13, which was still fast enough to qualify.
Six-time gold medallist Blake, who says this will be his last Games, nearly ambled over the line in the penultimate race of the heats despite an average start for his standards.
Usain Bolt made his grand Olympic entry with an easy victory in his 100m heat today. The Jamaican immediately set about his typical showman antics, with the raucous reception of his practice start drawing his applause.
"It wasn't the best start, I feel kind of sluggish".
Bolt's rival Justin Gatlin also won his heat with a gentle run that came through in 10:01. He is eyeing a triple-triple, having won the gold medal in men's 100m, 200m and 4x100m relay at the previous two Olympics. But he'll have to wait to see if he qualifies for the next round as a fastest loser.
Katarina Johnson-Thompson moved into the final medal position by jumping 6.51m.
Saturday's events include contests at various stages.
The 25-year-old finished seventh and was helped down the tunnel, but a successful appeal ensured she would appear in the final on Monday.
He qualified in a leisurely 10.07 seconds, picking up the pace after lumbering from the blocks and looking stiff in his warm-up. New Zealand's Mahe Drysdale and Croatia's Damir Martin crossed the finish line in the men's single sculls side by side, not knowing which one of them was the new Olympic champion.