If it helps, I have (very loosely) optimised my blog to whatever phrases I want my main website to rank for. Then, because I post about so many completely different and unrelated 'random' subjects, I optimize each post individually for one phrase I think people are likely to search for. You can probably tell by looking at the titles of my posts:
SP3 breaks Wireless Connection Manager in IBM Thinkpads (Lenovo)
Kitchens Direct Cold Calling
Ajax/XML problem with British Pound sign
Like I said, 'random' subjects! Which when you think about it, is what most blogs are like.

It's not sexy, but it works:
#1 - Google search for sp3 breaks wireless
#3 - Google search for kitchens direct cold calling (after my webforumz post since webforumz has more authority)
#1 and 2 - Google search for ajax pound sign

My point is. A blog, unlike a company website or portofolio, does not need to be associated with a correrent theme in order to rank well. Just do your SEO post by post as you go along...

Hope this helps.