You have a custom domain name which you have registered, and you would like to use this instead of: something.blogspot.com
So you would use www.yourcustomdomain.com which would automatically redirect to something.blogspot.com
I can see two ways of doing this, forgive me if there are more, which im sure there are. The first way involves modifying the CNAME record for your www subdomain of your registered domain name. Now this may sound like a mouthful, but all it means is that you need to go to the website where you registered your domain name, and if they support modification of domains and subdomains, then you would simply modify the www subdomain in such a way that the CNAME record points to ghs.google.com
Once you have done that, then you need to log into your blogger account, click on Settings -> Publishing, and enter your custom domain name.
And thats it, you can now use www.yourcustomdomain.com to view your blog. This method is however a bit of a problem when the website you registered your domain name doesn’t support modification of records. In this case, you would use an awesome Free DNS service known as DNS Afraid, and they enable modification of records. I used this method for pointing www.turkeytunnel.com to my blog. So to get it working, you need to go to the website where you registered your domain name, and you need to set the nameservers to NS1.AFRAID.ORG, NS2.AFRAID.ORG, NS3.AFRAID.ORG, and NS4.AFRAID.ORG
Once you have done this, go and create an account at Free DNS and then you click on Domains, and then add a domain. Enter the name of the domain you registered, and then click on manage. You will see a whole bunch of subdomains, and the one you want to look at is the www subdomain. Click on it, and make sure the record type selected in the drop down list is CNAME. Now enter the destination address as: ghs.google.com and you’re sorted! Of course this second method also involves setting your blogger account to recognize your custom domain (Described in paragraph 4 above)