Millipede - Brachyiulus apfelbecki
It was quite difficult to identify this cutie. Although not 100 % sure I have determined the species by the method of exclusion. From 8 species included in the Wiki page, 3 are endemic to Bulgaria. For the other species there is some very scarce information and species description, a couple of photos, but for this one - nothing.
Blackish color, with lighter yellowish central line on the back along the body, about 46-47 segments, about 45 mm long.
Spotted at Ognyanovo Dam, about 40 km from Sofia