media mogul Conrad Black (pictured) returns to the limelight by hosting a weekly talk show on Canadian television, his new employer says. ‘‘Lord Have Mercy, Black is Back!’’ shouts an advertisement for the one-hour newsmagazine talk show The Zoomer. Airing on VisionTV, it is aimed at older viewers. Black returned to Canada last May after serving more than three years in a US jail for fraud and obstruction of justice, convictions that completed a remarkable fall from grace for a man once courted by the rich and powerful. At its peak Black’s former newspaper group was one of the largest media empires in the world, and in Australia was a major shareholder in John Fairfax.