In my opinion, darkvision is an important enough trait that if familiars were intended to have it, it would have been called out in the description. Instead, the description of familiars is very specific on when they are treated as magical beasts ... "for the purpose of any effect that depends on its type." It would have been much simpler to have written "for all other purposes" or something similar.
For what it's worth -- not much, IMO -- when I wrote Skip Williams about my tamer of beasts' animal companions (which also became magical beasts), his reply was that they were only treated as magical beasts for the same purpose as listed for familiars above. (Of course, when I then pointed out that tamers of beasts would then be unable to do things like use speak with animals on his own companions, Skip added, "Except for the tamer, who can still treat them as if they were animals." So, like I said, not really worth much; even less, since that was under 3.0.)