I am just fine with Jackie Robinson to be the first.
Assuming there will also be others over time, I believe Sandy Koufax was the most dominating pitcher in the game, at his peak.
I'm not. Although he was a Dodger, he was also a Bruin. He played football for UCLA, but the teams he was on never beat the USC Trojans.
Sandy Koufax and Mr. Ed https://www.youtube.com/watch?v=AVm-HwAkVp8
Sandy Koufax and Mr. Ed https://www.youtube.com/watch?v=AVm-HwAkVp8