It is odd how the blazer exists in a no-man's land of formality between the lounge suit and the odd jacket, but there it is. Too casual for the suited office and too formal much of the rest of the time, the gilt-buttoned jacket comes into its own at sporting events and afternoon parties on the weekend paired with tan or cream trousers, chestnut shoes, a checked shirt and a casual necktie. And it is equally useful for cocktails and other late day occasions when accessorized somewhat differently with mid-gray trousers, a white shirt and black shoes.
Blazers come in both single and double breasted forms, and strangely enough the two have no common heritage. Single breasted is the more common version, which makes it safer but less interesting at the same time. Derived from the nineteenth century rowing club jacket the single breasted can be livened up a bit with an odd waistcoat such as a patterned tattersall in cool weather or a cream linen in warm.
Then there is the double breasted like the blazer in the photo. A descendent of the naval reefer jacket it is most often tailored, like its single breasted relation, from dark navy blue wool serge or hopsack. The double breasted is somewhat riskier to wear, having been tainted, in America at least, by the same situation comedies that convinced more than one generation that the ascot is worn only by ne'er do wells and villains. But that risk also means that the DB is better appreciated when worn properly.
Now not all navy blue jackets are blazers and the difference is the buttons. Navy jackets have an unfortunate tendency to look like a mismatched part of a suit unless accompanied by brown or metal buttons. And brown buttoned coats are technically odd jackets rather than blazers, though that is a fine distinction. The buttons on a true blazer are brass, silver or gilt. They are usually plain or with a subtle pattern, with the double breasted benefiting, in my opinion, from buttons with an anchor or other nautical theme in keeping with its heritage.
So that then is the blazer. It should probably not be the first odd jacket in a wardrobe. Tweed for example is useful more often and a suit will do on the remaining occasions when the blazer would be an option. But it certainly has a place as the second or third jacket in a season's wardrobe. Just don't wear it double breasted with an ascot.