I did a similar functionality to my blog. You shouldn't do a search and replace because there are a lot functionalities pointing to default.aspx and you might break more than intended.
Here is how I would implement your changes:
1. Copy default.aspx to blog.aspx. Don't forget to rename the class name in code behind. This would ensure that all the functionalities of the default page is intact. Then add your modifications to blog.aspx.
2. On the OnLoad event of the default.aspx, add this code: Server.Transfer("blog.aspx");
This way all the other codes referring to default.aspx would still work but the default.aspx page will take them to your new blog.aspx.
Hope it helps!