Like her slightly older predecessor Joni Mitchell, this Caribbean-born British folk legend has a way of making pop that’s both inviting and daunting. Her voice can sound high and bell-like one moment, booming and supple the next, and her stubbornly idiosyncratic songs demand way more than a cursory “Where’s the hook?” listen. But Armatrading’s best work casts a heady spell. The singer-songwriter hits NYC this week as part of her final large-scale tour.