Currently, StackExchange has the SF/F blog themed to look like the rest of the site - same color palette, same images, etc. We could attempt to mimic this look as closely as possible (we might be able to get the entire theme, this is still an open question) and the new blog will look basically the same as our current one.
Alternatively, we could attempt to use a more standard WordPress theme that also fits the main site's overall "feel" but with a different layout or structure. Or the blog could go a completely different direction. (Wordpress has zillions of themes to pick from plus we can tweak the CSS at will.)
Any opinions, please speak up!
(For reference, the "prototype" for an off-site blog is World Building. Their original blog design appears to be distinct from their site design, but word is they are attempting to incorporate more of the main-site elements into their blog. Take that for what it's worth.)