I think you are off to a correct start. To relate the insert on this page with the foreign keys for the userid and blogid you will need to have access to those id's. Since the blog should be associated with the user if you had the user id you should be able to create a recordset and query the blog table for the id.
You would normally have a user login set of pages. If you use Security Assist to create this set of pages you should have the user id stored in a session variable after the user logs in. You would then use this userID session variable to filter your recordset to get the blog id or id's for that user. Once you have the values you need from the recordset you can store the blog id in a session variable and make reference to this session variable for the insert of the blog entry.
Do you have a user login section setup on the site yet? If not I think this would be the next step. Also do you have an insert page for the blog itself?