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Black wire is normally negative and attaches to the - terminal of a battery but if you're asking where it goes then you're not seeing a missing wire from the battery..
Post photos, maybe someone will recognise your equipment and be able to help.
A black wire goes to the negative terminal, I would recommend getting some decent ring connectors as all the wires look like they are about totally frayed, only a matter of time before they all go, looking at them