In my opinion, the perfect blue blazer is one that is as versatile as possible.
You should first start with the fabric. A richer, deeper navy with some depth to the color should be preferred. Something perhaps in a year-round weight wool, depending on what your local climate allows. Such a suiting might not the most optimal for some regions, such as Singapore, so you should tailor this as you see fit. In addition, I would recommend a solid navy over something such as a herringbone. Or at least something that looks solid from anything more than three feet away. A solid, deep, rich navy will mean that you can work it in to as many ensembles as possible.
After that, there are the other details. I would prefer a 3/2 roll, but a 2 button could also work well, depending on your height and build. Given today's informal culture, two patch pockets and a soft shoulder would be useful in toning down the formality. At the same time, going with three patch pockets probably takes it a step too far. I would also recommend eschewing things such as obvious pick stitching (especially those ugly machine made ones that lower end Italian lines seem to all carry), peak lapels, hacking pockets, and even tickets pockets, just for the sake of simplicity.
Should you prefer something a bit more Neapolitan, you can get between one and three buttons on the sleeve (I would recommend three, just so you don't veer too far out). If you want something less provincial, I would recommend four.
Finally, I would recommend dual side vents, just because I think the jacket lays better when you're sitting down or have your hands in your pockets.
The key here is to create something that can be worn with almost any classic men's style that you might find yourself drifting into. It should be wearable with any combination of trousers and shoes, and be suitable for almost any climate. Of course, nothing really achieves all those things perfectly, which is why we have so many variations of designs and cloths. Still, if one could only have one blue blazer, I think the above would be best.
Edited by dieworkwear - 4/19/12 at 6:47pm