I suggest having +1 buttons affect whatever page they're on (i.e. individual posts instead of your homepage or blog page, if that's what's being displayed).
This maximises the chance of other +1 users seeing recommendations in their search results for the wide range of search terms that return your blog posts, instead of the handful of search terms that return your blog's homepage.
Having as many pages plussed as possible is a good strategy, because Google only gives up a small space in search results to show that friends recommended that page; the increased clickthrough from having 10 people the user knows recommend a particular site over one person recommending it is likely to be negligible. I will often visit sites that one friend has plussed, even if it's quite low in the search results; it really is a good indicator of quality. (If you trust your friends.)
Of course, the +1 button on your blog and homepage could still plus the index pages themselves as you've suggested.