Well I guess that depends upon your point of view. The Union Navy couldn't stop supplies from landing in Mexico and being transported through Texas overland to Richmond, until they bottled up the Mississippi River crossings.
Now there is no question that the Anaconda Plan wasn't 'leak proof'. There always were the lone flatboat which snuck across the Mississippi behind the Union patrol boats as they went down river. Or the occational Blockade runner that got into one of the southern ports in the dead of night, until all the ports were captured. But for the most part the floodgate of goods from Europe were cut off once the Mississippi was eliminated as a supply route for arms, ammunitions, artillery, cloth goods and beef cattle from the west in early 1863.