Sorry this is largely a stupid question; but since I'm not very sartorially experienced here goes: Is a navy or blue odd jacket more casual? (when I say blue I mean something in this shade http://ethandesu.com/post/34350034443/the-simplest-way-to-dress-blue-blazer-light)
Bunny - You need to do a bit more than throw out a half question - what is the context you are thinking of wearing stuff, what with , who with, when, why,how old are you roughly, what are you doing for a crust, where do you work, what do you imagine is the image you are trying to project etc - try and get up to a paragraph dude.
Because with what you are putting out - short brainfarts? - You doing this on a phone? the only answer is - it depends.
The simple answer is a blue blazer is one of the essential items in a grown ups wardrobe. Doesn't even matter what colour, or material, it can be cotton, it could even be a pea coat, or a "normal" blazer - its also versatile and a nice one may be as formal, with a tie and grey pants, as you may ever need to get in your whole life.