But the blockade was leaky and didn’t force Germany to the negotiating table before the damage had been done. Besides, my understanding (and correct me if I’m wrong) is that the blockade wasn’t actually intended to be a component of protracted siege warfare. The RN planners had just enough grasp of technology to realize that Copenhagening the HSF was a Bad Idea. The Distant Blockade was chosen over the Close Blockade because the British thought it was obvious that the Germans would have to come out and fight to break the blockade. Then the Royal Navy could enact dashing schemes like the Baltic Plan and Gallipoli* that would be real war winners.
Gallipoli wasn't a failure for want of naval support - the Allies threw an entire navy at it. It was just a crap idea badly executed.
The Germans didn't come to the negotiating table because they didn't care about starving people to death to achieve Final Victory - hence, you know, the Turnip Winters. Had the Central Powers been well-fed and well-armed the collapses of 1918 probably wouldn't have happened, so "before the damage is done" is a very teleological way of looking at it.
Of course the blockade was leaky. Blockades are leaky, because the sea is very big. Close blockades, historically, leaked as well, cf. the Napoleonic Wars (leaked terribly because sailing ships and weather) and the American Civil War.No blockade could crash the German economy quickly, but I think the RN always took a more cynical view of "the Allied armies will finish this by Christmas".
Conversely, if you don't have a considerable margin of superiority over the HSF, you can't actually fight the war at all - can't keep the constant flow of cross-Channel supplies, can't feed the civilian population, can't get at the enemy in the colonies - you can sit at home and starve. A Navy is not, cannot be, a war-winning instrument against an almost pure land power - the "almost" is what makes the blockade worthwhile, and the blockade was only effective at the margins because the Central Powers put such a lot of their manpower and resources into the Front. But not having the Navy when your economy and war effort rely completely on sea transport is certainly a war-losing instrument.