Rizzo had six hits in his last 56 at-bats, with 22 strikeouts. Overall, he had one home run, six RBIs and 15 walks in 34 games. He also struck out 36 times in those 98 at-bats.
"We felt it was time for Anthony to take a step back to take two forward," Black said. "Now it's time for him to go back and get his confidence back on track."
The Padres promoted Rizzo, who turns 22 on Aug. 8, from Tucson after he hit .365 with 16 home runs, 63 RBIs and a .444 on-base percentage in 52 games.
Rizzo reached base three times in his Major League debut against the Nationals on June 9, hitting a triple in his second at-bat.
Two days later, Rizzo, hit his first home run during a 2-1 loss to the Nats at PETCO Park.
But he struggled thereafter. He started Friday against the Marlins with a lot of friends and family on hand. Rizzo is a native of nearby Fort Lauderdale. Jesus Guzman started each of the last two games at first base, driving in six runs.
"This [struggling] is all new to me," said Rizzo, who has a .297 career batting average in the Minor Leagues. "This has never happened before.
"Now I've got to get my swing back. I know I'll be back up here."
Rizzo said that he felt his swing had gotten long during his time in San Diego.
"I'll get back to being short like I did the first week I came up here," he said.
Rizzo was one of four players obtained by the Padres from the Red Sox in the December deal that sent Adrian Gonzalez to Boston.
Blanks was hitting .351 in 34 games in Tucson, with 11 home runs and 35 RBIs, after he started the season with Double-A San Antonio, playing left field and first base.
Between the two stops, the 24-year-old Blanks is hitting .312 with 15 home runs, 52 RBIs and a .382 on-base percentage.
Blanks, the Padres' starting left fielder on Opening Day in 2010, saw his season cut short when he had Tommy John surgery on his right elbow on July 30.