Question: when did black widow first appear in mcu
[{'answer': 'April 1964', 'provenance': [{'wikipedia_id': '709344', 'title': 'Black Widow (Natasha Romanova)', 'start_paragraph_id': 1, 'start_character': 413, 'end_paragraph_id': 1, 'end_character': 423, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '709344', 'title': 'Black Widow (Natasha Romanova)', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 662, 'bleu_score': 0.7833763197108501, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10025375935851502401}, 'section': 'Section::::Abstract.'}]}]