Things changed — dramatically — for rapper Daveed Diggs after being cast in a not-so-little musical called “Hamilton.” His breakout performance in the dual roles of the Marquis de Lafayette and Thomas Jefferson earned Diggs critical acclaim, and soon after came the awards (Tony, Grammy), TV roles (“black-ish,” “Unbreakable Kimmy Schmidt,” TNT’s upcoming “Snowpiercer”) and film appearances (“Wonder”). Now comes “Blindspotting,” which opened Friday, in which he and longtime pal (and spoken-word artist) Rafael Casal star and make their feature film screenwriting debut.