You are right Bojan, this blue is unique in byzantyne iconography. Here are a few more info about it and about Voronet Monastery:
Voroneţ is one of the famous painted monasteries in Romania, found in southern Bukovina, now in Suceava County. Often known as the "Sistine Chapel of the East", the frescos at Voroneţ feature an intense shade of blue known in Romania as "Voroneţ blue", and even more “The misterious Blue Voronet” that still rest a mistery over the centuries.
The monastery was established in 1488, with St. George the Martyr as its patron saint. The exterior paintings were made in 1547. It remained a working monastery until the start of Habsburg rule in 1785, and only became a religious retreat again after the fall of communism in 1991.
The church is one of the Painted churches of northern Moldavia listed in UNESCO's list of World Heritage sites. (source: Wikipendia)