This is either Xylocopa violacea or Xylocopa valga.
Both species are common in West, Central and South Europe.
In eastern and southern Europe, Xylocopa valga is more common than Xylocopa violacea.
The males of both species are easy to distinguish under field conditions: in X. violacea, antenna segments 11 and 12 are orange (not the case on the photos) and segment 13 (apical) is black and characteristically curved, whereas X. valga males have black straight antennae (not the case on the photos).
I guess therefore the individual on my photo is a female.
A stereoscopic microscope is needed to study females of both carpenter bee species.
Their antennae differ with regard to the length of the base segment (shorter in X. valga) as well as the number and distribution of sharp minute denticles on the exterior surface of hind tibia (more numerous in X. valga).
Macin Mountains, Tulcea County, Romania, 22 May 2018