If you browse through this exchange of views
http://www.feldgrau.net/phpBB2/viewtopi ... c&start=15
you will find some interesting points by Ernesto Cernuschi about British blockade measures imposed on Italy before it actually entered the war in June 1940.
The point is that Britain imposed a blockade on Europe designed to prevent supplies reaching Germany through neutral states. Neutral states such as Spain were prevented from importing more than they needed for their own use, so as to prevent re-export to Germany.
Britain also undertook other measures against neutral countries, such as mining Norwegian territorial waters so as to deter the shipping of Swedish iron ore to Germany along the Norwegian coast.
It is true that Germany did not suffer the same food shortages that caused its Home Front to collapse during the First World War. But if you read the book I recommended, "Starvation Over Europe", you will understand why that was so. Germany simply satisfied its own food requirements from its own production and the production of the countries occupied by it, meaning that the burden of the 10% deficit in the total food supply of German-occupied Europe was diverted away from the German people and onto the peoples of the occupied countries.
Germany adopted a system of rationing the available food supply, ie the 90% of total requirements that could be produced within German-occupied Europe, based on the principle that other peoples would starve before the German people did. That principle in turn was derived from Germany's experience in the First World War, where its failure to ruthlessly exploit the territories it did occupy in Eastern Europe (in particular Ukraine from March to November 1918) resulted in starvation in German and more severely in its Austrian ally.
Not only did the food rationing system reserve available food supplies for the German people, so as to maintain its normal standard of nutrition, it also reserved food for population groups favoured by the German Government. The book "Starvation over Europe" shows that some peoples, eg the Danes and the Czechs, enjoyed a standard of nutrition equal to that of the German people and in some cases even better.
The other side of the coin was that less favoured groups received much lower rations. According to the book cited, the lowest rations were accorded to Jews and Greeks. It is understandable why Jews received the lowest ration, given their status as the most disliked people from the German Government's point of view, but it is hard to understand why the Greeks were equally disadvantaged. Perhaps that was a result of logisitical factors, and it is noteworthy that Germany cooperated with the Allies to supply food to the starving urban population of Greece.
Although a shortfall of 10% may not sound like much, it can be a matter of life and death when the burden of that shortfall is concentrated on a particular part of the population, ie if the available 90% is shared among, say, 97% of the total population of the blockaded area, giving it about 93% of its requirements, while the remaining 3% of the population receives nothing.
The crucial issue is that although theoretically Germany could import food from or through neutral countries such as Portugal, Spain, Turkey etc., Britain's command of the sea-approaches via the Atlantic and the Mediterranean meant that it could control imports by those neutral countries, reducing their ability to re-export to Germany.
Once Germany had occupied most of Europe, prior to the invasion of the Soviet Union, it became responsible for the feeding of that entire area, both itself, its allies, and the countries occupied by it. That is when the 10% food deficit of the European Continent (excluding the Soviet Union) really began to bite.
In short, one should not fantasise about large convoys of merchant ships arriving at Lisbon and Turkish ports loaded with food for Germany and its allies. That simply did not happen.
Much is made of the economic agreements between Germany and the Soviet Union of 1939, 1940 and 1941, which allowed Germany to import food and other raw materials, in particular oil, either from the Soviet Union itself or acquired by the Soviet Union from other countries for re-export to Germany.
However, it needs to be remembered that the Soviet union was not giving lend-lease aid to Germany. The three economic agreements were barter arrangements whereby the Soviet undertook to deliver fixed quantities of raw materials to Germany in exchange for Germany's delivery of fixed quantities of manufactured goods, including arms and other forms of war-production, to the Soviet Union.
It was very difficult for Germany to keep up with its deliveries, since they weakened Germany's own war-fighting capacity, and when they lagged the Soviet Union cut back its own deliveries in retaliation. For example, in the second half of 1940 Soviet deliveries were drastically cut back, on the excuse of Germany's tardiness with its deliveries, and they were not restored to their former level until the third economic agreement of early 1941. It is possible that the reason for the Soviet reduction of deliveries in 1940 was actually Stalin's shock at the unexpected sudden German victory in June of that year, and his resultant concern to weaken Germany.
The economic relationship between Germany and the Soviet Union prior to June 1941 was nothing like that between Britain and the United States, or Britian and its Commonwealth and Empire. Britain was never in a position where, in order to receive supplies of food and other raw materials, it had to divert a substantial part of its war production to a supplier that could well turn out to be hostile; it was always able to rely fully on its suppliers.
Finally, I would doubt that a German wartime piece of propaganda could be relied on to present a true picture of Germany's actual situation in the face of the British blockade.