"He comes up to me and says, 'We normally do this in my office, but you're going to be out there,'" Medica said.
The 25-year-old, who made his big league debut with the Padres last September, said he'd gotten hints he'd be on the opening day roster and had a good feeling when Black came over to talk to him.
"I was still out there, I was still in uniform, so I thought it was a good thing," Medica said.
He planned to tell his parents in person when they arrived at Fowler Park at the University of San Diego, where the Padres played the Cleveland Indians on Saturday afternoon.
"It's awesome to be on the team on opening day. It's a dream come true," said Medica, an infielder-outfielder.
The Padres host the rival Los Angeles Dodgers in baseball's North American opener Sunday night.
Xavier Nady, 35, who had been on the bubble, made the roster after left-fielder Carlos Quentin was put on the disabled list when a sore left knee.
Nady, who made his big league debut with San Diego in 2000, also got the on-field treatment from Black.
"It was a brief conversation. He came up to me on the field and said, 'Do I even need to call you in or do you understand that you're on the team?' '' Nady said.
"I'm excited for the opportunity and excited to be a Padre again," Nady said.
Nady, a first baseman and outfielder, has battled injuries, including having reconstructive elbow surgery, and was in Triple-A all last year and part of 2012 as he tried to keep his career going.
"Obviously every year your goal is make a big league ballclub," Nady said. "But it's been a grind the last couple of years. You go through things, whether it's injuries or still playing in Triple-A to see if you still have it. I came into spring and all I wanted was an opportunity and thankfully they gave it to me. I'm thankful to be here."
Quentin, who's been slowed by knee injuries the last two seasons, injured his left knee during spring training. He had surgery on his right knee in September last season, the third operation on the joint in 18 months. He's making $9.5 million this season.
Also starting on the DL will be right-hander Josh Johnson, who has a strained right forearm, and center fielder Cameron Maybin, who hurt his left (non-throwing) biceps March 2 when he robbed the Dodgers' Juan Uribe of extra bases with a diving catch. Maybin played in only 14 games last year.
The Padres also optioned infielder-outfielder Kyle Blanks to Triple-A El Paso.