What affects the shade of blue sapphire

If a sapphire is called "blue", then its secondary colors (e.g. green or purple) should not make up more than 15% of its total color. Learn more about blue sapphire at https://jupitergem.com/gemstones/natural-blue-sapphire. 

Sapphires that have more than 15% secondary colors are referred to by jewelers as greenish blue, purplish blue, violet, and so on. These are not "blue sapphires", but "unique", or "fancy color" sapphires. Stones of similar shades should be separately classified and evaluated accordingly.

Sapphires, like any minerals, acquire an additional color to their “own” color due to impurities of other elements in the crystal . Blue sapphires become, stained with impurities of titanium and iron. The more of these elements, the more the stone "turns blue". However, too much saturation can give a dark or dull effect to blue sapphire, which is undesirable and reduces the value of the stone.