Solid navy blue is the most versatile colour for a blazer. The more coarse the weave, generally the more casual the jacket.
Pair it with a polo shirt or a button-down collared shirt. Add a tie to latter if you want a more formal look. Go for a shirt in a contrasting colour to the blazer - pale blue, cream or white, for example.
Pair that up with chinos (more casual) or tailored trousers (smarter look) and maybe a pair of loafers or chukka boots if it's colder.
Hope this helps.