I switched to Yearly & still 404'ing.

I have two blog modules set up - one called News, the other Blog so don't think it is a naming issue.

Very peculiar.

Ok so here's how I got it working. The urls the archive widgets were creating were like http://mydomain/blogname/year/month. after reading this thread. http://www.silverstripe.org/blog-module-forum/show/7839, I realized what the $allowed_actions were doing which also allowed the calendar, which had a similar functionality to use a /view/year-month url. thinking someone would likely do the same for the blog so I looked at the $allowed_actions in /blog/code/BlogHolder.php and found one called "date" so I added that to the url and it worked. Then i found where the urls were being created and went into blog/code/widgets/ArchiveWidget.php and changed lines 90 - 94 to prepend the "date/" value:

if($isMonthDisplay) {
   $link = $container->Link("date/".$sqlResult['Year']) . '/' . sprintf("%'02d", $monthVal);
} else {
   $link = $container->Link("date/".$sqlResult['Year']);
}

this changed the archive url to http://mydomain/blogname/date/year/month and that worked. i'm not sure if someone built it in and forgot to update the widget or what but hopefully it saves someone else some headscratching time.