I'm not sure the water looks blackish because of pollution or simply because of its brackish nature. See below for the definition of Brackish.
Brackish water or briny water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty". Certain human activities can produce brackish water, in particular certain civil engineering projects such as dikes and the flooding of coastal marshland to produce brackish water pools for freshwater prawn farming. Brackish water is also the primary waste product of the salinity gradient power process. Because brackish water is hostile to the growth of most terrestrial plant species, without appropriate management it is damaging to the environment (see article on shrimp farms).
Technically, brackish water contains between 0.5 and 30 grams of salt per litre—more often expressed as 0.5 to 30 parts per thousand (ppt or ‰). Thus, brackish covers a range of salinity regimes and is not considered a precisely defined condition. It is characteristic of many brackish surface waters that their salinity can vary considerably over space and/or time.
The nature of the Baltic Sea
The Baltic Sea is a small sea on a global scale, but as one of the world's largest bodies of brackish water it is ecologically unique. Due to its special geographical, climatological, and oceanographic characteristics, the Baltic Sea is highly sensitive to the environmental impacts of human activities in its sea area or in its catchment area, which is home to over 85 million people.
What makes the Baltic so sensitive?
An almost enclosed sea
The Baltic Sea is only connected to the world’s oceans by the narrow and shallow waters of the Sound and the Belt Sea. This limits the exchange of water with the North Sea, and means that the same water remains in the Baltic for up to 30 years – along with all the organic and inorganic matter it contains.
The Baltic Sea consists of a series of sub-basins, which are mostly separated by shallow sills. These basins each have their own water exchange characteristics.
Runoff enters the shallow Baltic Sea from a large catchment area
At an average depth of just 53 metres, the Baltic Sea is much shallower than most of the world’s seas. It contains 21,547 km³ of water and every year rivers bring about 2% of this volume of water into the sea as runoff. The Baltic Sea’s catchment area is almost four times larger than the sea itself.
The brackish water of the Baltic Sea is a mixture of sea water from the North Sea and fresh water from rivers and rainfall. The salinity of its surface waters varies from around 20 psu (≈parts per thousand) in the Kattegat to 1–2 psu in the northernmost Bothnian Bay and the easternmost Gulf of Finland, compared to 35 psu in the open oceans.