This gonna be difficult for you. I haven't tried it yet but I hope you can get some thing from my suggestions.
Usually there is a user table in blog's database. It stores user's particulars and password. You will need to link your registration and login page to blog's user table. You will also need to check the login page of the blog. See what sessions it creates for logged in user. You will need to create the same ones in order for your logged in users to access blog.
You will need to check the database again. You will need a user_id (something like this) in your comment table. If the comment is submitted by a logged in user, store the user_id for the comment. If not, store a null value. When you display the comment, retrieve and display the user_id as well. Once you have the user id, you will be able to link to user's profile page.
This is not detailed enough but to give a rough idea. It will be too much to explain everything in one post.
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