Blue is ubiquitous as the color of choice for most sites commercial and professional. In the western world, dark blue is so strongly associated with strength and security that it is no surprise it is the color most favored by men. In Europe there are several religious associations with blue, but overall blue is considered the safest color to use worldwide when designing a website.
Blue is the color of the corporate world. When you see the deep rich blue, you know that you have found a website for a business that is trustworthy, secure and, I would add, in control.
Almost every website for banks in the United States either uses this blue as the primary color or sole color for its logo and website sending the message that yes, we will keep your money safe.
As do banks, most professional firms also use this blue. Accounting, law firms and commercial businesses tell you they are worthy of your trust and your business by using deep blue.
Blue also symbolizes male strength and the corporate mainstay of every business-person's wardrobe - the blue suit. Going on an interview with a professional organization? Wear a blue suit. Meeting a new client for the first time? Wear a blue suit.