I have been asked this question a whole lot of times and it appears a lot of people, especially men, do not know the difference between a Blazer and a suit jacket.
Many men actually go on wearing one for the other without knowing how, where and when to wear them.
I will highlight the difference between both pairs:
A suit always has matching trousers(pants) and is worn mostly with a shirt and tie, sometimes with a vest.. Suit are usually thinner with their fabrics and are be made of finer stuff like worsted wool. They also usually have a single, slits or no slits at all (openings on the sides or backs of jackets) and depending on the style, suit jackets comes in 2 or 3 buttons. The buttons on suit jackets are not obviously noticeable and blend in with the colour of the fabric. Most buttons are made of plastic, but sometimes the buttons are covered with the fabric used and the amount of sleeve buttons on a suit jacket always matches the number of waist buttons. Their pockets are sewn into the jacket’s lining and having an almost invisible opening and some less formal suit jackets have small flaps as pockets.
Blazers give a dapper look that is less formal than a suit jacket. These days, a blazer has come to mean anything that has sleeves and lapels and is worn with unmatched pants/trousers and comes in much thicker or heavier fabrics such as tweeds, velvet and linen.
Years back, jacket makers made blazers much different from the sport coats and the suit jacket but as fashion advanced, the blazer has fallen into the sport coats category and are made in the same design as the sport coat for example the tweed suits. Blazers these days are made out of all sorts of material. Tweed fabric blazers although not necessarily, are best worn in the winter periods.
The blazer buttons act as noticeable accessory, they don't always blend with the colour of the fabric. Most jacket makers use simple brass buttons, but many blazers display buttons of gold, silver or pearl. The pockets are sewn in different ways, some have patch pockets or flap pockets but the left breast pocket always remains open.
How, where and when to wear?
These days suit makers tend to make suit jackets with thicker fabrics but with matching pants/trousers. Once it comes with a matching pants of the same fabric and colour, then it is a suit jacket!
At many social functions, anything goes, however, some occasions require adherence to appropriate levels of formality. Wear suit jackets for formal events and leave the blazers for less formal occasions.
While no specific colour defines a suit jacket, dark or non bright colours have remained the best choices over many years, but you can find suits in every imaginable colour. As a general guideline, choose non bright colours for business, executive and formal events, and save brighter, vibrant colours for fun. You don't want to turn up in your executive meeting in a complete red, green or yellow suit as you would look completely out of touch.
Blazers, unlike suit jackets can be worn in different colours at any less formal event as long as the colour is compatible with the dress pants. For example a red blazer matched with a black pants, a white shirt and a tie or just rock a blazer with a casual T-shirt and jeans. When wearing a blazer for informal business or a social event, let the formality of the occasion determine what to wear with the jacket.
Don't ever try pulling off those pinstripes suit jackets as blazers. It's also an absolute NO NO wearing your suit jacket with a pair of jeans. Suit jackets are meant to be worn as suit jackets, with matching pants!