(Reuters) - London's blue-chip index ended in the black on Thursday as trade concerns were soothed by a two-week U.S. tariff reprieve on Chinese imports and Morrisons jumped on upbeat profit and forecast.The FTSE 100 index was in and out of negative territory through the session but ended 0.1% higher, boosted by a 1% rise in tobacco giant BAT after layoff plans that offset losses in oil majors BP and Shell . The mid-cap index , meanwhile, dipped 0.1% after scaling its highest level in nearly..