In WWI, the most important part of the "blockade" was to take advantage of the fact Germany had already declared war on its largest trading partners. All the UK had to do, therefore, was use its far larger international influence to keep the rest of the world trading with them rather than Germany, and keep Germany bottled up.
The biggest impediment was the US, who demanded free passage for all, and who the Entente needed to buy massive amounts of raw materials from. Luckily, the Germans proceeded to mistreat the Dutch and Belgians and mine the coast off of Southern England, which gave the British an excuse to turn the screws and institute a full blockade.
Simply put - the Germans couldn't beat the blockade, and even when they could, often they'd find the British either already bought what they needed or they'd find whoever helped them got crushed by Britain's outsized influence in international trade (aided and abetted by the US and the rest of the Entente.