The feminine form is indeclinable. According to ridni.org's 2013 study, <span class="searchmatch">Чорнобай</span> is the 1,272nd most common surname in Ukraine, belonging to 3,214 individuals...
Ukrainian Wikipedia has an article on: Чорнобаївка Wikipedia uk From <span class="searchmatch">Чорноба́й</span> (Čornobáj) + -їв (-jiv) + -ка (-ka), from the surname of Cossacks founding...