There are bloggers who have converted their blogspot blog to custom domain, but then changed their minds and want to switch back to blogspot. Previously, this was very easy done. One just need to go to Settings > Basic > Publishing and there will be a visible link to reset it back to the blogspot sub-domain.
Blogger, being fond of torturing bloggers now with their new UI (User Interface) have for reasons only known to themselves hidden it.
Now you will still need to go to Dashboard > Settings > Basic and in the section for PUBLISHING and, pay attention, click the obscure X to delete the custom domain and presto, the blog will revert to its previous blogspot sub-domain. Why on earth Blogger do not display the blogspot URL for us see and easily know what to do, why, that is just beyond me.