Hi guys, this is my first post; and as a DM vocalist (who can do some pretty good BM too) I thought this would be a good place for it.
Anyways, I'm not sure about Blind Guardian (I haven't given them a really good listening yet; but I have their 'Nightfall in Middle Earth' album) but Alestorm uses normal vocals with just a bit of rasp. Not really growling or screaming.
If you want to learn to do vocals for metal, the best place to start is with the "sigh technique". Just do a sigh or two; you'll notice it's very similar in sound to metal vocals. Now just try to get that "Umph" you're looking for in there. Then, start saying words and take it from there.