On “Preach,” the opening cut of Blk Jay’s Blkout EP, the Duval-rapper testifies that “They’re gonna feel this: the reign of Black.” Over the course of the release’s short-but-tight collection, he backs up that testimony. Vibes sui generis crackle through Blkout. For one, there is the evident quality of restraint. Jay keeps the momentum at a lax first-gear: “F’sho” is …