As always, start from Settings - Publishing, and select "Switch to: • Custom Domain". If you were buying "mydomain.net", for "What address would you like your blog to have?" you should simply enter "mydomain", select ".net", then hit "Check Availability". This month, if you do that, you will see
There was an error processing this form. Please try again.
URL should end in a valid domain extension, such as .com or .net.
In this case, you must enter "mydomain.net", then select ".net". The ".net" bit is validated, too. If you enter "mydomain.com", then select ".net", you'll get the same error.
Strange, but true.