Navy Blue Blazer

A: Navy blazers are a great wardrobe stretcher since they are in and of themselves a pseudo-neutral article.
The one thing you need to remember though, is that navy and black are not great pairings. The end result usually looks as though someone mismatched a suit. Try to keep a balance by using lighter colored pants and skirts and soft-colored blouses (or turtlenecks) in order to keep things looking flattering.
The exception to this is the combination of navy jacket and navy shirt with a tan or khaki pant, which works well and is a classy, professional look without being too dressy.

See also:
How not to look fat in jackets
Layering clothes: How to do it right
How to mix and match clothing
Matching your hairstyle to your clothes