The classic preppy version has brass buttons, even on a fabric like that.
Seriously, why do so many people here hate brass buttons? They're fine. Desirable, in fact. The navy blazer with brass buttons is the classic, the version without them is the derivative variation. Flannel is perfectly fine, and a classic winter variation.
And I agree with those saying that in the US, a blazer is just a step below a suit. A flannel version would be a little less formal than that, but still perfectly appropriate to wear to an office where suits are worn most days.