Your question is wrong, You should be asking why so called European nations such as ,Portuguese, Italians, Greeks, Maltese, Cypriots are darker? Bulgarians have a very complex ethnic heritage, First being proto bulgars, slavic, thracian (probably greek with some balkan influence) . Proto Bulgars are a tribe moved from the north of black sea to the current region of Bulgaria, It is debatable but proto bulgars are said to have an Indo-Aryan heritage with some old proto turkic influence with their military. And more for the latest historic influences Bulgarians are not dark because of the Ottoman Turks. If you think that bulgarians are dark because of the Turks, then rest of the Balkans would be the same, but it is not. In Bulgaria you can see very dark skinned person, also very light skinned, you can see whiter Bulgarian Turk than Bulgarian or lighter Bulgarian than a Bulgarian Turk. Roma population is similar to Indians so they are generally darker than Europeans or Bulgarians. Do not forget that Ottoman Turks were never actually same proto Turks from Central Asia, most of the proto Turkic ethnicity was assimilated with the local Anatolian ethnicity being greeks,armenians,kurds,georgians and others like persian,arabs. So in that sense, Turks does not have that much effect on Bulgarians from the Ottoman times. Adding to your questions most of the mediterranean nations are more or less same, they have the same genetic influence from the past, in that regards if you think that greeks are lighter than Bulgarians, you might be wrong.