The North could have lost the war only if Grant and Sherman could have been stopped. If Sherman takes Atlanta and marches to Savannah and Grant pushes Lee into the trenches, the war will be won. For the blockade to have been the decisive factor, one must show how it could have prevented the success of Grant and Sherman.
In addition to the capture of all of its useful ports, the South had the problem of not having enough cotton available in the runner ports to pay for the goods that did make it through the blockade. If more cargoes made it in, the need for cotton from Alabama and western Georgia would have been greater, but the railroads did not have the rolling stock to move the cargo as it was.