I just built a new 5wt rod kit from Hook and Hackle that had green thread included for a green blank. Like you, I wasn't too keen on the monochromatic idea, so I ordered chestnut brown thread at the same time. It was worth the extra bit. I did use the green for the ferrule wraps and I didn't use any color preserver on them. The result is that the ferrule wraps pretty much vanished when finished. If the blue thread is dark enough, it might work for you as well if you don't want the ferrule wraps to be prominent. It's a matter of personal taste and preference. At any rate, don't compromise on the color...get what you want and it is worth it.

By the way, dealing with Hook and Hackle was a good experience and I'll be using them again.

For convincing your wife, pick up some Hardy literature and leave it around for her to see, then once she sees the prices you wait a little bit and then reveal your revelation...I can build a rod for a fraction of the cost! If you time it right, she will be so happy.