A Code Black was a transmission made by the Obsidian Order in the mirror universe.

In 2377, the Obsidian Order made a Code Black transmission to Intendant Kira Nerys (who was actually Iliana Ghemor posing as the deceased Intendant) when the IKS Negh'Var arrived at a rendezvous with an Alliance battle fleet at Regulon. (DS9 novel: The Soul Key)

Given Ghemor's familiarity, the primary universe Obsidian Order might have used Code Black transmissions as well.
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