I could reasonably identify it as Mordellistena parvula or Mordellistena pumila. The three species are indistinguishable based on photographs. All three species are found in Bulgaria. I did not find any detailed descriptions on the internet, only a few photos and almost all of dead specimens.
''Mordella aculeata'' is a species of beetle in the ''Mordella'' genus that is in the ''Mordellidae'' family, which is a part of the ''Tenebrionoidea'' superfamily. It was discovered in 1758.