In every PSU I've looked at inside, all the black wires went to the same copper area on the circuit board. All wires of the same color are like this, except if they're the voltage sensing wires that are paired up with another wire (usually thicker) in the same connector pin (almost always one of the wires paired up with an orange wire, and it may be orange or brown, but I've seen a sense wire paired with a +5V red wire and even with a black ground wire), but I speculate that some high power supplies split some wires of a given color into two groups, each with its own current limiter (I've read that yellow +12V wires can be like this).