Do the blogs follow the same linking structure? Will you be mergin posts into the same categories?
Just 301 the old domain to the new site. Go through your website logs to see which sections of the old site gain the most search engine traffic and use htaccess to again 301 permanent redirect the old urls to the new location.
Read this blog post here for all the info you need:
http://www.seobook.com/archives/002343.shtml