Shortest route, Sofia, Belovo, Yolunda, Yakaruda, Banya....
Crap road through Rhodope mountains... Thats the way sat nav will take you...
As Wen says, E79 to Blagoevgrad through Dupnitsa, turning at Simitli off E79 and on up to Bansko. Once out of Sofia road good..
You could consider Sofia, Samakov, Dupnitsa and on, 25km longer, but no need to go through Sofia...