the second one is very common, but for me it doesn't matter any of those will do its just depends on how you promote it and how many quality backlinks you can earn for that blog.
if possible,choose yoursite.com/blog .as people visiting your site will come and visit your blog..many blog visitor will follow-through to your site,but it doesn't matter any more which format you will choose.
Hi there,
Yoursite.com/blog is much better then others. This kind of URL take much importance in SEs. Because I find most of time this kind of blog URL in top of the search engines.