Here's yet another helpful video from Mr Porter about that staple of any man of style's wardrobe, the blazer. Often the term is used loosely nowadays to describe any dark jacket, rather than the traditional navy or yachting-inspired dark blue coat with shiny buttons.
I try to avoid being a fashion policeman, but please don't go for a look designed to make others think you've just stepped off your private yacht. It's a bit sad to have a blazer with a badge on the breast pocket and fake rank stripes round the cuffs (unless you're a real naval officer or an official at the Royal Yacht Squadron). I joke not, there's one leading men's fashion site selling just an abomination as we speak.
Incidentally, I love the word blazer - it's thought to have been named after HMS Blazer, whose crew was rigged out in jaunty blue and white jackets by a proud captain in the days before Royal Navy uniforms were standardised. Anyway, blazers have moved on from the shapeless polyester sacks with horrid fake-brass buttons we bought in the eighties. Richard James has some calm and invaluable advice for you here.