Since my post this morning about the three earthquakes in the Ukraine side of the Black Sea, one more happened today at 16:10:56 (UTC).
Here is the map with all four events:
I also found a tectonic map with the major fault zones at the end Jurassic (after Robinson et al, 1997), that may shed some light regarding the seismicity in the Black Sea. It appears that the lone event is related to a fault zone bounding the Shatsky Ridge in the eastern Black Sea, and the three clustered ones SW of Sevastopol are near the Jurassic tectonic boundary of the Tavrik-Kure basin.
Low magnitude earthquakes are not uncommon in the Black Sea. Below is a list of all the 2010 seismic events with a magnitude greater than 2 on the Richter scale (from the National Institute of Earth Physics, Romania):