With the demise of the old Balkan Airlines flag carrier they became the largest airline in Bulgaria. Fleet mainly of Tupolevs, they expanded recently by taking some 146s.
They manage a number of other fleets as well. The Balkan Holidays aircraft you see in the UK and across Europe are operated by them, as are Albanian Airlines, which gave them their introduction into 146 operations.
Like most Eastern European operators you will probably find the flight crew are ex-Air Force jocks.