The tournament uses a modified format, where a player needs to win four games to clinch a set, and winning one point at deuce is enough to take the game.
Tien, who won his first trophy on the tour at the Moselle Open last month, held his nerve and made only 12 unforced errors while Blockx had 23, as the American won the match in just under an hour.
"I'm really happy. I knew it was going to be a tough match. I don't think (Blockx) missed a serve for the first set-and-a-half. He's been playing great in these conditions all week," said Tien, who lost last year's final to Joao Fonseca.
Blockx, who served seven aces while his opponent had only one, made his intentions clear from the start, attacking the left-handed Tien's backhand with a fast serve and running up to the net to apply pressure.
Tien, ranked 28th in the world, pushed Blockx back with a well-placed lob before winning the point with a drive volley and although Blockx, ranked 116th, saved a break point and pushed the first set into a tiebreaker, top seed Tien outplayed his fellow 20-year-old.
Tien won the second set with a decisive break, using his powerful forehand to push Blockx back until the under-pressure Belgian second seed hit over the baseline.
The American, who has clinched five victories over top-10 ranked players this year including a straight-sets win over world number two Alexander Zverev in February, got another break in the third set to go 3-1 up, leaving Blockx with little chance of fighting back.
"What a year you've had," Blockx told Tien at the trophy presentation ceremony. "There are not a lot of days I feel helpless on court but today was one of them. You're just too good."