The blue blazer is a staple in any man's wardrobe. This article of clothing is so versatile that it is inconceivable that the investment would not be worthwhile. You could literally make hundreds of outfits out of this one article of clothing.
Below are the top three types of looks that can be created just from the pairing of the blazer with other clothing.
The Dressy Occasion. Let's say that you are going to a day wedding or perhaps out to a fancier restaurant for dinner, you could put this piece with a pair of khaki pants, a button down shirt and a cool tie.
If you arrive at the restaurant and the dress code is more lax, you can always remove the tie but,especially in cases where you are unsure of the attire/dress code, this outfit is always a good bet.
Meeting the Folks. Let's say you are going to meet your girlfriends parents or even your future in laws at their house for a "relaxed" dinner. While a button down and khaki pants look nice, they give the impression that you only half tried.
If you put the blazer over that outfit sans tie, then the impression is that you care enough to add a little special touch. Additionally, as the dinner wears on and everyone becomes more casual with each other, you can remove your jacket.
A Night on the Town. For most women, nothing is sexier than a man in jeans with a blazer. It just gives them a look that says, "I'm professional but I can relax." Besides, on a cold night, you could really impress the girl by giving her your coat.
Business casual events are another great place for the blue blazer. Many times companies send their employees off for training and the event calls for business casual.
For many people this means that they'll end up wearing a polo shirt or a button down shirt with khaki pants. They might even have to opt to wearing their suit pants with a button down shirt.
The first scenario is a little too casual in my opinion. It makes you look like you are managing a shoe store in the mall. I've managed shoe stores in the mall, and although its an honest profession, I don't want to feel like I'm at the mall when I'm training for a more professional job.
Unfortunately, the second scenario really isn't ideal either. You just end up looking like a poorly dressed person in a suit. When you're wearing a suit, wear the suit in full. Otherwise, its better to just go with the blue blazer.