There is some information at:

http://www.divingtravel.com/bulgaria.htm
http://www.goldensands.bg/padi/default.htm
http://www.scubayellowpages.com/destinations/europe/bulgaria.htm
http://www.dogsbreathdivers.com/Bulgaria.htm
etc.

J?ri

Andy:

 From:

http://www.sochiclub.narod.ru/blacksea.htm

"Many people still wonder why it is called "black". The color of the
sea depends on the weather. On cloudless days the sea is blue-green
and when the sky is covered by heavy, dark clouds it is silvery gray
or azure. But no one has ever seen it black.

   Ancient Greeks called it Pontus Euxinus (Hospitable Sea). In the
9th and 10th centuries AD it was known among the Russians as the
Russian sea. There are several hypotheses of the origin of its present
name. Some historians say that it is a translation of the Turkish
Karadeniz, which means "black, inhospitable sea". The Turks and other
conquerors of its shores associated the sea with the bitter resistance
by the local population.
Another theory comes from hydrologists. The name, they assume,
reflects the Black sea's unique ability to tint black metal objects
that get deep down into it - the action of strong concentrations of
hydrogen sulfide. The sea surface gets intensely heated by the sun,
and has no time to cool down during short winter, so the surface water
scarcely penetrates lower down to the depths. These deeper waters,
because of absence of oxygen flow from the surface, contain no life."
 
...

"An unusual feature of the Black Sea is that oxygen is dissolved only
in the upper levels of its waters, which alone can support a rich sea
life as a result. Below a depth of 230–330 feet (70–100 m) at the
centre and 330–500 feet (100–150 m) near the sea's margins, there is
no oxygen because the sea is permeated by a high concentration of
dissolved hydrogen sulfide, forming a saturated, "dead" zone
inhabitable only by specially adapted bacteria. Despite this anomaly,
the Black Sea's uppermost waters supported abundant sturgeon,
mackerel, and anchovy until the late 20th century, when the runoff of
industrial and municipal wastes into the Danube, Dnieper, and other
feeder rivers caused increasing levels of pollution and consequent
reductions in fish populations."

...

"In ancient Greek myths, the sea—then on the fringe of the
Mediterranean world—was named Pontus Axeinus, meaning "Inhospitable
Sea." Later explorations made the region more familiar, and, as
colonies were established along the shores of a sea the Greeks came to
know as more hospitable and friendly, its name was changed to Pontus
Euxinus, the opposite of the earlier designation. It was across its
waters that Jason and the Argonauts set out, according to legend, to
find the Golden Fleece in the land of Colchis, a kingdom at the sea's
eastern tip (now Georgia). The Turks, when they came to control the
lands beyond the sea's southern shores, encountered only the sudden
storms whipped up on its waters and reverted to a designation
reflecting the inhospitable aspect of what they now termed the
Karadeniz, or Black Sea.

To scientists the Black Sea is a remarkable feature because its lower
levels are, to all intents and purposes, almost biologically dead—not
because of modern pollution but because of continued weak ventilation
of the deep layers."

Hoping that this helps,
Daryl Krupa