Does Black Market Broker apply to hidden cards, since they are face down?

Ruling: Yes, Black Market Broker applies to hidden cards because hidden cards are played from the facedown zone.

Sequence:

• Hidden cards are put in the facedown zone associated with the battlefield they are hidden to
• You play hidden cards from that facedown zone
• Black Market Broker's "face down" text applies to this zone

Nuances:

• "Hidden" itself is not a zone; it's a mechanic that places cards in the facedown zone
• The templating uses "face down" rather than "hidden" to be more technically accurate about which zone the card is played from