Most applications use blue as their central color, particularly business applications (e.g. Windows uses blue in most of it's chrome). I am being pushed to use purple throughout our current application (because that is the color of the business logo) but I think it looks more professional in shades of blue, so I need to argue my case (more eloquently than saying 'purple sucks'). Is the preference for blue perhaps a cultural thing or physiological thing (i.e. less taxing on the rods/cones in the eye)? Why is blue preferred over purple?
Blue is the color of communication.
Blue is also a color that isn't effected by color blindness, or at least not as often as red-green.
"Blue, often associated with stability, symbolizes trust, wisdom and confidence. Blue has been shown to produce a calming effect and is often used to promote products and services related to cleanliness. More accepted by males than females, it is a preferred color for corporate America."
From http://www.pallasweb.com/color.html: "•Purple: creativity, mystery, (reddish purple) royalty, mysticism, rarity. Purple is associated with death in Catholic cultures, as mentioned above. •Blue: loyalty, security, conservatism, tranquility, coldness, sadness. Light blues create a feeling of openness, clean air and freshness, while dark blues can convey tradition, trust and solidity."
It all depends what you want to convey with your site!