Black spot grants you an additional ranged or melee attack after you kill a model that is marked. You do not need to directly hit the model to get the free attack so lobbing cannoneer AOEs into stealth models still gets you an extra attack if you kill one.
Here is the main thought I had: If you kill a model with a melee attack Black spot gives you a free melee OR ranged attack. Now picture this, a your target unit has black spot on it. You give rush the the Cannoneer for speed and you have it charge. You kill the model with black spot with a melee attack and now you get a free attack and you choose the range option and fire and a new target.
This is a great threat range the people will not see coming. You can get the cannon shot deeper in their lines than they thought was possible and kill off some key support pieces that they thought were out of range.
Base 4+ charge 3+ rush 2+ cannon 12= 21 inch max cannon reach
This could also be done with the other ranged models we have and even include snipe and/or eyeless sight.