The illustration shows us how to wear a summer blazer. The jacket itself should be a lighter hue of blue than winter's navy and in a breathable fabric like fresco or linen. Three buttons is best, rolling to 2.5, and there should be patch side pockets so the jacket can have the minimum of interior lining. Personally, I like sterling silver buttons for a change of pace, with two of them on each sleeve.
Wear the jacket with light gray fresco trousers and brown and white spectator shoes. A panama hat tops off the ensemble with flair.
And there you have the summer blazer. If you can manage to be tall and thin, so much the better.