The singer, songwriter and painter Joni Mitchell (“Both Sides Now,†“Blue,†“Court and Spark,†“Hejiraâ€) and the drummer/bandleader Brian Blade, one of her frequent collaborators, talk with The Times’s chief pop music critic, Jon Pareles, about Ms. Mitchell’s work and life on the occasion of a concert at Massey Hall in Toronto, “Joni: A Portrait in Song,†in celebration of her 70th birthday this year.
