Dear Ashwini,
Can I know the job profile which you are looking for?
The blue sapphire as the name suggests comes in blue color. Blue sapphire with black shades is known as kaka neelam which is not fit for astro purpose. And to avoid any cheating while buying get, always buy a certified gemstone from GII or IGI.
It is good to know that you worship Lord Shiva. Keep it going!
GOD BLESS!!!
Srinivasa Temkar