What is a black ace?

Excuse me, could you please clarify what you mean by "black ace"? Is it a term related to cryptocurrency or finance? If so, I'm not immediately familiar with it as a standard industry term. Could you provide some context or explain how it's used in the field? Alternatively, is it a reference to a specific strategy, tool, or concept in the world of cryptocurrency or finance? I'd be happy to help explore the meaning and implications of "black ace" if you could provide a bit more information.