Assuming this map is accurate, the areas it highlights are those in which rivers flow into the Black Sea. A number of things about this may perhaps be geopolitically noteworthy:
– some countries, most notably Ukraine, lie almost completely within the Black Sea basin, whereas others, most notably Poland, lie just beyond the borders of the basin. Others still, like Bulgaria, are neatly bisected by it.
– a number of important cities, such as Istanbul, Lviv (the largest city in western Ukraine, with the exception of Odessa), and the capital cities of Bulgaria (Sofia), Belarus (Minsk), and Bosnia-Herzegovina (Sarajevo), are situated precisely at the outer edge of the basin. Ankara, the capital of Turkey, is almost at the outer edge of the basin, meanwhile.
– the easternmost areas of Ukraine have rivers that flow into Russia’s Don River, and a coastline located along the Russian-controlled Sea of Azov rather than along the open Black Sea. Given that eastern Ukraine is relatively hilly (which this map does not show well) and is a leading producer of bulk goods like coal which are expensive to transport via truck, this Russian-oriented riverine and maritime access of eastern Ukraine is especially notable.
– The two most important rivers within this basin, namely the Danube and the Dnieper, empty into the Black Sea only about 200 km apart from one another, as a result of the fact that the Danube turns sharply north just before it reaches the sea, while the Dnieper turns southwest.
Yes, I try not to think about it when I eat fish in Istanbul 😦