Sitemaps are not generally necessary.
For most sites, sitemaps are only used by Google to ensure that they can effectively crawl your site by comparing the results of a crawl with the sitemap. Where a sitemap is necessary is for extremely large sites, sites that cannot be effectively crawled, sites with a paywall or login.
Sitemaps do nothing for SEO.
If your site can be properly crawled, you do not need a sitemap, however, if you do provide one, do not confuse Google and make it complete. It is possible that you can use both sitemaps by using an index sitemap. An index sitemap points to other sitemaps. You can get the information on index sitemaps here: http://www.sitemaps.org/protocol.html#index
In your case, you would generate your separate sitemaps, name/rename them appropriately, then use the index sitemap to reference the files created. You can very easily hand edit your index sitemap.
What happens if you do not include your blog sitemap?
Nothing. Google will compare your sitemap against what they find in their crawl. However, if Google finds more pages than what exist in your sitemap, it may discount your sitemap entirely. Otherwise, the only cost is effort. You will be wasting your time and effort.
Remember that if Google can effectively crawl your site, it will continue to do so. If there is no difference between a crawl and the sitemap, the sitemap is largely ignored.
Again. Sitemaps are not necessary. Yes, I know that half the planet recommends one, but it is false advice parroted around out of ignorance of how a sitemap is actually used.