Blockades and embargoes are notoriously slow to be effective, and in a rural society like the American South during the Civil War, in a time that was less dependent on imports anyway, this was even more true.
The blockade was important, no doubt, as it denied the ability of the Confederacy to sell cotton, mainly to England, and to purchase important goods for the war effort. Seeing as how southern industry was very limited at that time, they needed raw materials and especially weapons and food in order to have a chance in that war.
Blockade runners were moderately effective at keeping a trickle of supplies coming in, while they ran great risks and made great money for their efforts. However, almost 50% of them who went out never returned to port.
The economic pressure the blockade placed on England, for a time, looked like it could backfire by provoking British intervention on behalf of the South, so it was risky to pursue such a policy. By the time the British had decided against coming into the war (1863) the blockade was indeed beginning to bite. Prices for consumer goods in the South rose, cotton languished in southern ports, and the war effort continually struggled to fund and supply itself against a mammoth invasion from the North.
So the blockade was effective, to a degree, but it was only a small ingredient in what was necessary for a Union victory.