It looks like the Greek colonies along the northern shore of the Black Sea played an important role in intermediating the wheat trade to the Athenian empire during the Peloponesian war. Did something happen in later ages that this trade stopped? Did Pontic wheat become uneconomical or not available any longer to the classical world?
Well, it didn't. The trade from the Black Sea to the Mediterranean, of which grain was a crucial component, continued well into the Roman period and beyond. In fact, it was a crucial aspect of Byzantium, and later Constantinople's, success. During the Hellenistic and Roman periods, Greek colonization continued, and the colonies themselves founded. I believe our best information on them is from the Hellenistic, in fact.
But the other question is whether, in fact, Athens was feed by Sythian grain. Our source for such an importance is the Athenian orator and politician Demosthenes, who, among other things, was famous for securing the good relations between Byzantium and Athens. It was thus very much in his interests to magnify the importance of Black Sea grain imports. A careful study of other, more specific evidence has shown that the grain was not nearly so crucial or constant.
There is a good discussion of this in a chapter of Patterns of the Economy in Roman Asia Minor.