The Violet Carpenter Bee (Xylocopa violacea) can easily be confused with the Common Carpenter Bee (Xylocopa valga).
Both species are common in West, Central and South Europe.
In eastern and central Europe, Xylocopa valga is more common than Xylocopa violacea because Xylocopa valga prefers warm, dry sites or those moderately humid.
In western Europe in Atlantic coastal areas with high air humidity, Xylocopa valga is replaced by Xylocopa violacea.

I am not sure whether the photos here show a Xylocopa violacea or Xylocopa valga.

The males of both species are easy to distinguish under field conditions: in Xylocopa violacea, antenna segments 11 and 12 are orange (not the case on the photos) and segment 13 (apical) is black and characteristically curved, whereas Xylocopa 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.
The females' antennae differ with regard to the length of the base segment (shorter in Xylocopa valga) as well as the number and distribution of sharp minute denticles on the exterior surface of hind tibia (more numerous in Xylocopa valga).

Macin Mountains, Tulcea County, Romania, 22 May 2018