The Great Oxygen Catastrophe, sometimes called the Great Oxidation Event, was a devastating event that occurred about 2.3 billion years ago.
Recent scientific discoveries indicate that there were periods with elevated oxygen levels even earlier (about 3 billion years ago).
The "key organisms" for this early development of oxygen in the atmosphere are, according to current doctrine, the precursors of so-called cyanobacteria. Cyanobacteria are anaerobic organisms; they produce oxygen as a toxic waste product of their photosynthesis.
Anaerobic organisms do not need oxygen to live.
During this time, the color of the earth changed from black basalt to rust red.
Slowly but steadily, and then ever more rapidly, the free oxygen in the atmosphere rose to critical levels.
The increased oxygen levels in the oceans possibly killed off a large portion of the anaerobic organisms that populated the Earth at the time.
For them, it was a deadly poison, so the oxygen caused what is probably the largest mass extinction in Earth's history.
In fact, the oceans were saturated with iron in the 2+ oxidation state (Fe2+) before the major oxidation state.
However, because it is soluble in water, they were green.
As the oxygen content continued to increase, Fe2+ became further oxidized to Fe3+, which is not soluble in water.
It was deposited on the seabed as iron hydroxide 3+.
So, if the oceans are blue today, it's because of the oxygen catastrophe.