The only scenario when Black Market keys can be spent without player's participation is if it's the end of the Black Market season. If you have any spare keys by the end of the season, they will be automatically converted into a loot prioritizing most valuable chests. If you have fewer than 10 keys, these will turn into 1 Bronze chest. For example you have 1823 keys left. By the time Black Market closes it will bring you items from:
— 1 Golden chest (1000 keys);
— 8 Silver chests (8 x 100 keys);
— 3 Bronze chests (2 x 10 + 3 keys on top).