OK, lets simplify this, is the wire from the black boxy part connected correctly on same connection as the blk (ground?) wire?
My guess would be no. I also don't think that the black wire from the regulator is a ground (typically ground wires are brown on an Alfa).
My guess is that the "black boxy part" is simply a capacitor to suppress RF noise coming out of the alternator into the big, red wire. The noise suppressor is grounded by it's mounting screw and its black wire goes on the same stud as the big red wire. You might wait until someone else confirms this, but I'm 98% sure that is right.
Alternatively, leave "black boxy part" unconnected and see what happens. My guess is that everything will work fine, except that your radio will buzz at a frequency that goes up/down with engine rpms.