It is clear even before Mary J. Blige appears for her cover shoot on a balmy Thursday morning in Brooklyn that the singer is her own weather event. A gust of nerves and angst passes through the airy salon. Not even 100 percent of the correct vibes—glowy mirrors, purple patent swivel chairs with the sheen of hard candy, and walls bedecked with fellow icons like Macy Gray and Lil’ Kim—can assuage the jittery crew. In the back of the salon, the singer’s go-to stylist, Jason Rembert, is…