The main focus of a blazer is to create a professional look. Whether you're going to an interview or just going to work, a blazer is a must in most cases. People who don't know your role at work may assume you're the manager or someone who plays a vital role in the company based on your look. As far as an interview goes, the interviewer will get the impression that you take yourself and this opportunity seriously. So your first blazer should be classic, professional and preferably black.
Your second blazer is more relaxed and fun. This blazer allows you to create new and refined looks from your wardrobe. It's also a great way to transition your summer tops and dresses into fall. You'll get a lot more use out of your clothes this way. Some blazers have a colored lining so when you roll up the sleeve, you add a pop of color to your ensemble. For those of us who love leggings, a longer blazer is perfect for covering your derrière and goes great with your riding boots.
Your last blazer is trendy and appropriate for going out. It's the piece that replaces your jacket when it's too cold for sleeveless pieces. Your jacket isn't dressy enough so you throw on that blazer for warmth and style. Black is another good choice if it's got good details like this BCBG Max Azria blazer but if you can buy one in a color that matches cohesively with your look, it's going to intensify your style.
FASHION TIP: Although a blazer is a part of suiting, don't confuse the two. Depending on the nature of the job, it may be a better fit not to wear the dress pants or skirt that are paired with the suit, as it may give off too much of a corporate vibe, that the company is not attempting to portray.