The blazer has always been the uniform of seriousness. This season's best ones have lost the stiffness and kept the authority: softer shoulders, a longer line, pockets you can actually use.
Wear it over a fine knit with straight trousers, or over a white T-shirt with jeans. The rule is simple: if it fits at the shoulders, everything else can be negotiated.
Buy fewer, better. A good blazer, worn for ten years, is the most sustainable thing in a wardrobe.