How to redirect the old blog domain to the new one

See this: Blogger Help: Import and export blogs

To make a copy of your blog's posts and comments, you have to export your blog and then import it to another existing blog.

Export Your blog

To export your blog, simply click "Export Blog" from the Settings | Other tab.

Then, click the Export Blog button. Your blog will be stored as a Blogger export file (.xml) file which can be kept as a backup on your hard drive or imported into another blog. Exported blogs are not deleted from your dashboard or from Blogger.com.

Import Into an Existing Blog

To import posts and comments into an already existing blog, go to Settings | Other and click on "Import blog" from the "Blog tools" section. Don't worry -- your template won't be affected by imported content. Next, choose a Blogger export file (.xml) from your hard drive and fill out the word verification beneath. By default, all of your imported posts will remain unpublished until you publish them from your dashboard later. However, if you would rather have all posts published immediately, make sure to check the box next to Automatically publish all imported posts before you click Import Blog.

If you didn't choose to have your posts publish automatically, you'll need to manually publish them from the Posts | Imported tab before they will appear on the web. You can either publish all the posts at once by clicking the checkbox at the top and selecting all before clicking Publish, or simply checking the ones you want to publish one by one.

Imports and exports are only for posts and comments. If you want to save a copy of your template, click the Backup/Restore button on the Template tab.

You can redirect your blog to a custom domain using blogger. Here, what happens is that all hits your blogspot domain will be redirected to your new domain.

www.olddomain.blogpsot.com/existing-page.html to www.olddomain.com/existing-page.html

This can be achieved by setting up the Cname and A name records.

If you have a new domain, then you can set it as your new redirect as :

www.olddomain.blogpsot.com/existing-page.html to www.newdomain.com/existing-page.html

But you can't redirect from www.olddomain.com/existing-page.html to www.newdomain.com/existing-page.html using blogger settings. This is because the domains are not handled by blogpsot.

Either you should check for options with your domain provider to set up a re direct or point your old domain to a new blog and set a redirect to new url (Give a link to new url and notify users that you have shifted the domain).