ISTR the origin of Cyril and Methodius (the two monks that created the precursor to the cyrillic alphabet, can't remember the exact name of that alphabet ATM) cannot be pinpointed as either Greek or Bulgarian because no such documentation has been found. They surely were Byzantine, though.
Cyril and Methodius didn't create Cyrillic. That's a very common misconception. They created the Glagolitic[1] alphabet which was a precursor script. Cyrillic was developed later by their students and other scholars in the Preslav Literary School[2]. They named it Cyrillic to honour the brothers, but the brothers themselves didn't create Cyrillic.
EDIT: Sorry, I misread what you wrote. It's late and it's been a long day. You weren't saying the brothers created Cyrillic. As for whether they were Greek/Bulgarian I cannot say. I've read different opinions on that throughout the years. Definitely Byzantine, but anything else I cannot say.
