While the British blockade impacted a number of critical supply lines, the one that would decisive in the long run would be the food supply.

Like many participants in the First World War, Germany (as well as Austria Hungary) was not self-sufficient with its food supply. Food shortages would set in from the start of the blockade, and, despite numerous efforts to improve the situation, Germany was never able to properly address the issue of food shortages.

This issue was exacerbated by unfortunate events during the war. Early in the war, the Russian invasion of Galicia left a significant portion of the Austro-Hungarian food supply in enemy hands, and even after the region was recovered, the devastation caused by the fighting meant that they'd never be able to recover to their pre-war output over the course of the war. As tended to be the case as the war went on in every area, Austria-Hungary became increasingly dependent on German aid when it came to feeding itself, but not too surprisingly Germany continued to prioritize itself. As a result, by the end of the war, rations for residents of Vienna were living on starvation rations. With Vienna tending to get better treatment than other regions in Austria-Hungary, one can only imagine how bad the food situation was elsewhere in Austria-Hungary.

While Germany seemed to avoid the level of disaster that unfolded in Austria-Hungary, it still suffered from critical food shortages in spite of their best efforts. Early efforts to deal with shortages involved the typical "ersatz" replacements and additives to stretch supples - tea and coffee were replaced with ground up nuts or weeds with additives thrown in for flavoring, less-desirable foods became more commonplace, and, as supplies became critical, flour would be mixed with sawdust to stretch what was available. Things became desperate in the winter of 1916-17, when poor weather in autumn devastated the potato crop. At the same time, shortages of manpower were impacting harvests, so critical food shortages set in as the winter rolled around. This period became known as the "Turnip Winter," where the population was forced to resort to consuming turnips - something that was typically reserved for animal feed.

You mention the occupied Russian lands. That was something the Germans tried hard to exploit to make ends meet. The regional administration, known as Ober Ost, did everything they could to squeeze every last resource out of the region. The local population was reduced to very minimal rations and locals were conscripted for various labor projects. Food was requisitioned to send home to Germany proper, and extraction of resources was generally performed with minimal concern as to the long-term viability of such efforts. Poland, for example, had its lumber resources largely devastated as the Germans aggressively harvested it during the war.

Now if all that sounds familiar, that's because the German policy in Ober Ost was a precursor to the policies it would have in the East under Nazi occupation. However, unlike the Nazis, these efforts didn't have a genocidal intent, and, while they were certainly devastating to the regions they impacted, feeding the local population in Ober Ost remained a begrudging liability. In fact, despite the aggressive efforts to extract food and other resources from these occupied regions, most of Ober Ost was never truly self-sufficient with food either - instead being a further drain on German resources.

If you're looking for a resource on the subject, I'd recommend checking out Ring of Steel: Germany and Austria-Hungary in World War I by Alexander Watson. It's the source I'm primarily drawing from and it has lots of hard numbers that detail the economic reality of the war effort of the Central Powers - specifically, how precarious the situation was and how it only worsened over the course of the war.