Apatin has been located in Serbia since after the end of WW I, around 1918. That is where records will be located. Germans in that region, who settled there in the 1700s after the ouster of the occupying Ottoman Turks, are known as Danube Schwabians.
The surname BLASEVITCH may be a Germanized Croatian name, which would be BLAZEVIC, from the male given name Blaz from St Blaz or St Blasius. And although these folks were Germans Janos is a Magyar name. But this area had a mixture of people, Magyars, Germans, Croatians, Slovaks and others. I have found it not unusual for Croatians and Germans to intermarry in those days in that region.
If they were Roman Catholic then you are in luck! As there very best verifiable records will be church records. And... the LDS Church (Mormon) Family History Center near you will be able to order microfilm church records which you may rent for $3.50 at a FHC near you ! Here is what they have:
Roman Catholic Church register of births, marriages and deaths for Apatin, Hungary, now Apatin, Serbia. Text in Hungarian and Latin.
Title Anyakönyvek, 1826-1895
Authors Római Katólikus Egyház, Apatin (Main Author)
Kereszteltek 1826-1860 FHL INTL Film
Kereszteltek 1861-1870 FHL INTL Film
Kereszteltek 1871-1880 FHL INTL Film
Kereszteltek 1881-1890 FHL INTL Film
Kereszteltek 1891-1895 Házasultak 1826-1860 FHL INTL Film
Házasultak 1861-1895 FHL INTL Film
Halottak 1826-1855 FHL INTL Film
Halottak 1856-1870 FHL INTL Film
Halottak 1871-1880 FHL INTL Film
Halottak 1881-1895 FHL INTL Film
I'll check both the Serbia and Croatia phone book if you like to see if those names exist. However the majority of Germans were expelled or killed after WW II in that part of Serbia.
Croatian Heritage Museum