Oh, I saw this photo provided by my breeder (they have a section explaining all types of sheltie colors and include pictures) of a double Merle so I thought it just meant they were mostly white with a little merling-so perhaps this wasn't the best picture to represent what a DM is then huh?
http://www.canamishelties.com/images/cassiewhitepic.jpg
(I think this link is okay? There's no access to their site from this link..let me know if it's not allowed!)
DM's can have varying amounts of white and blue, just like a regular merle. Generally, they'll have a white or mostly white head, though (just the opposite of a CHW).