As for flaws, they themselves are things of beauty. They provide variation. It's the flaws in a blue diamond that give it the beautiful blue color (ie. electron holes).
Tune into Discovery Health Channel to see the beautiful "flaws of God" ...
No, it's the boron.
Type IIb: These diamonds contain no nitrogen - but they contain boron, which absorbs red, orange and yellow light. These diamonds therefore usually appear to be blue, although they can also be grey or nearly colorless. All naturally blue diamonds belong to Type IIb, which makes up 0.1% of all diamonds.