The wire is solid black and is not bundled with the blue sheathed wires, it should have a double wire connection on it. originally the wire went from your tach to the negative terminal on your coil, there it had a double wire going to a single spade connector, from there the other wire in pair went to your distributor, if you have pertronics that wire is unplugged and probably set aside somewhere as it is no longer needed, try to find that black wire and plug it into your negative coil terminal.
if that wire is already plugged into your coil then check the back of the gauge and make sure teh single black wire is plugged in there as well. the tach is a pretty simple piece of equipment to troubleshoot, it has power, ground, and signal (the black wire) check each of those and if they are all present when the key is on then the gauge may be faulty.