Seems you can use the blog's url to get the comments as well, try;
http://someusername.blogspot.com/feeds/comments/default
And you can get all the comments for a given post in the similar way, namely /feeds/xxxxxxxx/comments/default, where xxxxxxxx is the post ID.
And you can qualify by date by using the query string ?published-min=2010-03-16T00:00:00&published-max=2010-03-24T23:59:59
As a side note, the Blogger API can return the feed as JSON rather than XML, which I found way easier to deal with than parsing XML by using the query string ?alt=json