Assuming this is a conventional points/distributor system:
Don’t know whether you are referring to the wire that feeds coil from ignition switch or the wire that goes from coil to points. If it is the latter, the wire could well be black as it will be “earthy” each time points close. If points happen to be open at moment of measuring voltage then the black wire will be floating at +12 volts with respect to earth, as will the other incoming wire.
