Sorry for the slow response; I haven't been using DA much these days. I just used heat to blue this helmet the low-tech way. I use a small propane torch (really small, like the one you would solder copper pipes with) and heat it very slowly, moving around the helmet and bringing the temperature up evenly and slowly, so that hot spots diffuse out. Eventually the colour will come up through straw, purple and then blue. Get it too hot and it will turn a less appealing blue/grey.
Vendel is actually a place name. It was a village in the Swedish royal estate, and the prevalence of 7th C. royal burials there has led historians to apply the name to the era. So "Vendel" means 7th C. Scandinavian.