The black gold you describe is leachate, this is water that hasn’t evaporated during the hot composting process. Instead it drains down to the bottom of the HOTBIN, dissolving humic compounds from the compost as it does. This forms the dark brown liquid – leachate which is basically compost tea.
You should be getting around an espresso cup amount full a week - you can collect it via a diy method and use it as a fertiliser if you wish.

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