You can take the or better and more luxary are the buses from Greece via Kulata to Sandanski/Blagoevgrad/Dupnitza and from there you can find buses to Samokov, 9 km away from Borovez, there you can take the Minibuses to Borovez for a lev or a taxi for 7 to 10 levs.
The price from Sandanski to Samokov will be around 7 levs.
The trip needs around 3 to 4 hours in Bulgaria.
Sorry Nic, can't tell you there you can find the bus plans from greece to bulgaria, may be you can find it in the internet, the timetables for bulgaria you can find at the bigger busstations there. All is a bit older. Or try to find a train, from greece to Dupnitza and then take the bus from there.