Romania only introduced surnames in the 19th century, so if a Romanian surname has a non-Romanian origin there's great chances that at the time it was registered that village where it appeared was inhabited by people of that ethnic background. in this case we focus on Bulgarians. we already know the Romanians called Sîrbu mostly descend from a Bulgarian on the paternal side, as Bulgarians were called Serbians in Wallachia until Romania got independent from the Ottoman empire, because otherwise these Bulgarians risked being deported back south of the Danube.

this is only a small inventory of the most obviously Bulgarian surnames of Romanians, while many others are harder to trace, for example those formed from common religious/church-related names that were common to both Romanians and Bulgarians. but the way the diminutive is formed in Bulgarian is totally different from Romanian, which points to these surnames originating in Bulgarian common first names (not patronymics). I was able to check three names in the list with southern Romanians holding them and they all confirmed they knew of Bulgarian origin but also all of them confessed they previously thought it may have been inaccurate, as their father's ancestral village doesn't speak Bulgarian anymore, and were happy to hear the names are indeed Bulgarian. the presence is mostly along the Danube, on the northern shore. this leaves aside the Bulgarians in Dobrudja, which often did have a surname/patronymic ending in their name (-ev, -ov), as Dobrudja was taken very late/recently by Romania so they may have had already their surnames fixed in documents in Bulgarian standard by that time.

Личо (Liciu)
Нечо (Neciu)
Данчо (Danciu)
Станчо (Stanciu)
Енчо (Enciu)
Стоян (Stoian)
Банчо (Banciu)
Билчо (Bilciu)
Генчо (Ghenciu)
Гойчо (Goiciu)
Гочо (Gociu)
Денчо (Denciu)
Дечо (Deciu)
Дончо (Donciu)
Кънчо (Canciu)
Нанчо (Nanciu)
Начо (Naciu)
Неделчо (Nedelciu)
Панчо (Panciu)
Райчо (Raiciu)
Стойчо (Stoiciu)
Тончо (Tonciu)
Цочо (Țociu)
Петко (Petcu)
Станко (Stancu)
Банко (Bancu)
Бранко (Brancu)
Данко (Dancu)
Динко (Dincu)
Иванко (Ivancu)
Минко (Mincu)
Панко (Pancu)
Тинко (Tincu)
Цанко (Țancu)
Чинко (Cincu)
Дико (Dicu)
Вълкан (Vîlcan)
Драгой (Drăgoi)
Радой (Rădoi)
Боян (Boian)
Ванко (Vancu)
Цано (Țanu)