OK, I just did this again works like a charm. Here's what I did, using phpMyAdmin:
1) Make a copy of existing 2.4 database (you really only need these tables: sitetree, sitetree_live, blogentry, blogentry_live).
2) With your database copy, you delete the following fields from sitetree and sitetree_live: ToDo, Priority, Status, HomepageForDomain
3) Export the records with class name 'BlogEntry' from sitetree and sitetree_live. Also, export the blogentry, blogentry_live tables.
4) import them into your SS3 site!
A few things to note: you will probably need to change the 'ParentID' value of the BlogEntry records to match the ID of your BlogHolder page unless coincidentally they match, otherwise the blog entries won't show in CMS or on site! Also, upon importing into your new site, you may run into a Primary Key conflict if there are pages already existing with the same ID as something you are trying to import. In that case, all you need to do is change the ID of the record you are trying to import to something unique.
Hope that helps.