My understanding is that prior to the migrations/invasions of the Goths, Huns and Sklavenoi into the Balkan peninsula, Greek and Latin where the most common native languages, alongside other steadily declining languages such as Thracian and Illyrian. Also, I'm aware of the "Jireček Line" which is the theoretical line that divided the Balkans into the areas with Greek influence from those with Latin influence
My question is, do we know what was the most northern native Greek speaking colony in the Balkans? I'm assuming such a place would be located on the coast of the Black Sea? What about inland Balkans? (Also, I am aware of the status of Greek and Latin as lingua franca's during Rome's imperial period, but I'm mainly concerned with the use of Greek as a native language.)