Well, Wolverhampton is by far the largest town/city in the vicinity!  The MBC areas of Dudley and Sandwell (and Walsall) contain several large towns within - Wolverhampton MBC does not.  The location of Wolverhampton is generally reasonably well-known so defining the rough geographical location of the Black Country as the area to the south and east of the city (without including the city) makes sense, even if Dudley is the largest town within the actual Black Country.