The Dagorhir realm of Eryndor has provided a number of great blue weapon builds. Hail Eryndor!
Some of these guides have some dated techniques or materials.
Bluesword (swf) — This Guide is one of my favorites. It is very well put together, clean and thorough. The only downside to the guide is that it suggests PVC for a core. PVC is acceptable for first time or quick and dirty builds, but for a weapon that will be around for a while, fiberglass is the preferred core. For a one handed sword 1/2″ round fiberglass rod is good.
EVA Bluesword (pdf) — I like this guide for how thorough and concise it is. It also suggests the use of higher quality materials (fiberglass and EVAlite). There are only two flaws with the guide. Primarily, all of the color coding can be a bit overwhelming initially. Second, the guide suggests duct tape on the striking surface of the weapon. In my experience, a well constructed weapon shouldn’t need any tape to hold it together and duct taping along the flat of the blade is more for reinforcement. I have seen foams shear along the edges of tape (bluefoam can be especially notorious for this) on striking surface.
Stabby One-sided Bluesword (gif): — I have to admit, I’m not particularly fond of this next guide. Double-coring (when two small diameter cores are used) is an acceptable method, but not ideal. Furthermore the guide suggests the use of an open cell foam for the stabbing tip. Modern stabbing tip technology uses either Yoga-mat or Marinefoam (aka Ensolite) (Ensolite can also be found from seat cushions). A discussion of stabbing tip foam can be found here. Lastly, I dislike the profile of this weapon. Due to the use of open-cell foam, the sword needs to be reinforced along the flat to prevent the tip from blowing out.