Question: when did metallica's black album come out
[{'answer': 'August 12 , 1991', 'provenance': [{'wikipedia_id': '18760', 'title': 'Metallica (album)', 'start_paragraph_id': 29, 'start_character': 28, 'end_paragraph_id': 29, 'end_character': 44, 'bleu_score': 1.0, 'section': 'Section::::Commercial performance.\n'}]}, {'provenance': [{'wikipedia_id': '18760', 'title': 'Metallica (album)', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 909, 'bleu_score': 0.9300094393996641, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11900941005289331159}, 'section': 'Section::::Abstract.'}]}]