On the first day of each month (at 00:00 UTC), the Black Market resets, and all unspent keys are automatically used to open chests. The timer displayed under the Superchest progress bar shows exactly how much time is left until the reset, so you can always check the remaining time in advance.

Please also note that keys automatically used during the reset do not contribute to the Superchest progress bar or Leaderboard objectives.

You can find more information about how the Black Market works here:
https://warrobots.com/en/posts/337 

If you have any questions related to the Black Market, please create a request from this article.