You know though, that an identical WordPress.com site will always outrank a self-hosted WordPress.org installation. The SEO here is the best there is, and you also get to share in the incredible search engine ranking of WordPress.com, and that is nothing to sneeze at.
Secondly, the search engines no longer pay any attention to meta tags (keywords) since they have been so over used and abused. The only thing they may (and that is a tiny "may) use is a meta description, but in my experience with all sorts of sites, there is only about a 40% chance that a very well crafted meta description will be used as a snippet in search results.
In general the search engines pull their own keywords out of the content and that goes for the snippets they use in their search results as well.