It's not really necessary, I have one, but I don't think it is useful for a blog. Pinging services are enough.
In blogger, there is already a sitemap. You need not to do anything. just add /atom.xml after your blogspot url.
I really don't know if having a sitemap on blogger helps, they have an RSS feed that you could use. Besides, from what I know blogger blogs are optimized for google.
The difficulty with a sitemap on blogger is that there's no place to post it. The best you can do is create a sitemap using a sitmap generator and then posting an external link. I think your best bet is create an rss feed and submit the feed. For blogger its usually the URL of your blog followed by /atom.xml