though I've never been to Bulgaria, Sofija has that sort of East-West mix...
In my opinion, and I've often thought about the Syldavian inspirations, both Bulgaria and Albania seem the most likely candidates that inspired Herge's creation of Syldavia.
For me, I agree that the Cyrillic signs and letters suggest Bulgaria, and also fits the Turkish styled mosques and obvious Turkish influences in peasants dress style (male fez, womens headscarves and long full skirts, etc), and King Muskar's similar looks to Bulgaria's King Boris III.
But apart from the Cyrillic clues, I personally think there are as many clues to suggest Albania as an equal or even bigger inspiration for our Balkan Syldavia.
The predominance of a Muslim-looking peasant majority population, seemingly more Turkish inspired mosques than churches in every town and village scene, an equally strong likeness between King Muskar and Albania's King Zog, the comparisons between Syldavia's "Pelican" flag/emblem and Albania's black double eagle flag/emblem.
Other clues outside of the Czech (Bohemian) and Romanian (Iron Guard) ingredients, that make me think of Albania that little bit more than Bulgaria (or even Serbia) are the recent 1939 Italian Facsist invasion of Albania (similar to the Bordurian pressure) and the previous hostile threats behind the scenes from Italy to King Zog and Albanian independence, and the general feel of Syldavia'a Oriental/Ottoman appearance (more mosques than churches, peasants costume, etc) fits Albania's majority Muslim population far more than Bulgaria's minority Muslim population.
Am I missing any other clues?
So all in all, for me its both Albania and Bulgaria - But somewhere in between the both, that is my Syldavia.