The Wikipedia article on the Black Sea begins by asserting that “The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia…” This definition is not helpful, as it obscures more than it reveals.
Let us begin with the assertion that the Black Sea is a marginal sea. Wikipedia defines a “marginal sea” as follows: “A marginal sea is a division of an ocean, partially enclosed by islands, archipelagos, or peninsulas adjacent to or widely open to the open ocean at the surface, and/or bounded by submarine ridges on the sea floor.” The Black Sea, however, is not even remotely “widely open to the open ocean”: the distance from Istanbul, near the entrance to the Black Sea, to Gibraltar, near the opening to the “open ocean,” is 1,871 miles, or 3,011 kilometers. In the map posted below, I “erased” the Mediterranean Sea to illustrate the separation of the Black Sea from the ocean. Classifying the Black Sea as part of the Atlantic Ocean strains credulity. The very concept of “marginal sea,” moreover, is itself strained. As the second map below shows, many bodies of water that are officially classified as marginal seas are not “partially enclosed by islands, archipelagos, or peninsulas.” None of the officially delimited Antarctic seas have any real degree of surface enclosure. Nor are they divided from the open ocean by “submarine ridges on the sea floor.” 
The definition of a “mediterranean sea” (note the lower-case “m”), unlike that of a “marginal sea,” is clearly formulated and technical. According to Wikipedia, “A mediterranean sea is, in oceanography, a mostly enclosed sea that has limited exchange of water with outer oceans and whose water circulation is dominated by salinity and temperature differences rather than by winds or tides.” The same article lists eight “mediterranean seas,” which I have mapped (see below). It also divides this kind of sea into two categories: “concentration basins,” which are saltier than the open ocean, and “dilution basins,” which are less salty (see the second map below). Note that the Mediterranean is itself split on this basis, with the Adriatic forming a dilution basin and the rest of its waters a concentration basin. As these two different forms of “mediterranean sea” are situated on opposite sides of the open ocean regarding salinity, it might be more accurate to label a dilution basis an “anti-mediterranean sea.”
One thing that is clear is about the Black Sea is that it is not part of the Mediterranean Sea. The Wikipedia cited above, however, implies that it is: “The Eurafrican Mediterranean Sea is also a concentration basin as a whole, but the Black Sea and the Adriatic Sea are dilution basins …” Elsewhere in the article the Black Sea is classified as part of a plural entity dubbed the “Mediterranean Seas”: “The namesake Mediterranean Seas, including the Black Sea, the Sea of Azov, …” Evidently, a certain degree of confusion clouds these categories.
In geo-historical terms, the Black Sea belongs in a category of its own, as will be explored in the next GeoCurrentspost.