Fc
The distribution panel negative is direct to the batteries and the bilge, etc., negatives are run back direct to the panel (unless the PO did something very odd.) (for instance I ran a #8 "boat negative" that I tied those things into.) But the tach negative is thru the harness #10 black wire to engine ground, nothing to do with the panel. Again this all presumes that there's no "PO Job" done to the boat and that you have a good connection at the battery negative cable (bolted to the bell housing -- which should be relocated from there to a starter bolt.)
If you have the Westerbeke Captain's Panel then bets are off, it had some extra cabling for alarms and I'm not sure if it originally had an ammeter or voltmeter. The orange may have been an Alt out wire to an ammeter.
Just FYI, my tach signal wire was orange, not gray as most are.
The extra black may be (1) negative for the blower behind the panel, or to the diesel lift pump.
Oftentimes I use the pos and neg post on the tach as a "bus" to distribute to the gauges because it is heavier and handy to get to, and out of the way of the panel mess. But even so disconnecting that neg should in no way affect the bilge pump, etc.
-ken