Brian Boyle and Sasha Chmelevski also scored and Cal Peterson made 27 saves for the U.S., which won its seventh straight game in the tournament.
German provided a quarterfinal stunner with a 3-2 victory over Switzerland in the second of the four quarterfinals. In late games, it was Russia vs. Canada and Finland vs. the Czech Republic.
The U.S. was the top seed coming out of the preliminary round. It will play the lowest-seeded team remaining in Saturday’s two semifinals.
Boyle, Blackwell and Garland scored in the first period to stake the Americans to a 3-0 lead.
Peter Cehlarik got Slovia on the board in the second period, but Blackwell got his second of the game later in the period. Chmelevski and Garland closed the scoring in the third against goaltender Adam Huska, who faced 33 shots.
The comeback win over Switzerland sent Germany to the semifinals for the first time since 2010.
The Germans pulled their goaltender in the closing minutes and got a game-tying goal from Leon Gawanke with 44 seconds left in regulation. They won the shootout 2-1 when Marcel Noebels beat Leonardo Genoni.
It was another rough quarterfinal loss for Switzerland. Two years ago, Canada tied the Swiss with a second to play in regulation and eventually lost. This time they blew a 2-0 lead on goals by defenseman Ramon Untersander and Fabrice Herzog.
Tom Kuhnhackl cut the Swiss lead to 2-1 in the final three minutes of the second period.