When I opened my mail this morning I found a question from a friend: "do you know what the difference between a blazer and a sportcoat is?" Hardly a surprising question given my renown both as a clotheshorse and as a collector of trivia, fashion and otherwise. I replied, "Yes, I do. The question I think you wanted to ask is, 'what is the difference between a blazer and a sportcoat?'" Here's the difference.
A blazer generally refers to a single-breasted sportcoat (typically) in a solid color, usually bright — blue, red, yellow, green — but not always; there are blazers in blue, black, pastel, wine, etc. These days, a blazer is always solid, but the originals often had stripes. Some blazers have a crest on the pocket for a school logo or, for a while, a trendy fashion designer's logo. Naval officers, current and former, often wear their ship's crest. (Each ship of the line had a different crest.) While the origin is often claimed to be from "blaze" meaning bright, allegedly derived from the Cambridge crew team's bright red jackets, this explanation is quite wrong.
The name comes from a visit by Queen Victoria to the shop of the line HMS Blazer in 1837. At the time, sailors were a rather scruffy lot, as there were no uniforms or dress codes, and the Blazer's captain wanted to make a good impression. (It was, after all, the queen.) So he had short jackets made for his crew using a blue serge with brass buttons with the naval insignia.
The queen was so impressed with the sailors sartorial splendor that the jacket spread to other ships and then to the general public. This is why we still talk about "navy blue" blazers, and why for many years the slang term for a sailor was a "blue jacket." (The navy also invented bell-bottom trousers because they could be rolled up for sojourns among the rigging.)
Blue, by the way, comes from indigo dyes, the first natural dye that was reasonably color fast. Indigo was really the only option for stable dyes until Perkin, while searching for a means to synthesize quinine, synthesized mauve from coal tar and ended up a very, very, very wealthy man.
I'll spare you the gory details about pocket styles, lapel widths, fabrics, double versus single breasted, etc.
A sportcoat is any jacket that isn't part of a suit or formal wear. A morning coat or frock coat, for example, is not a suit component, but it is most certainly not a sportcoat. I don't know if a Nehru jacket would be called a sportcoat; I would call it a total fashion disaster.
I'm sure this was more than you wanted to know.
So, the short answer to the question is: All blazers are sportcoats, but not all sportcoats are blazers.
You gotta wear the blue blazer when you go to the big dance.
— Al McGuire, coach of Marquette, 1977 NCAA Basketball Champs, in response to reporter's inquiry if he would be wearing his lucky blazer. This is how the NCAA Tournament received the name the "Big Dance."