There are two main reasons why the Paris Guns couldn't be used for bombarding the British blockade. The first, as you mention in the question, is range. The British blockade was a distant one, operating in the North Sea between Norway and Scotland, and in the western approaches to the Channel. Both of these were well out of range of the Paris Gun, operating about 3-700 km from German-held territory. The Paris Gun was also far too inaccurate to hit a ship. The guns were just about accurate enough to hit Paris at their maximum range. Hitting a smaller target with any reliability was impossible. Ships would also have been particularly difficult to hit, as they could manoeuvre in the time between the shell being fired and it hitting.