Transition was Blogger, allowing that even with Blogger / Google controlling the domain authoritative servers (which they did, indirectly, by using eNom / GoDaddy as the registrars), and the initial DNS address setup (offered for GoDaddy purchased domains), the stability of custom domains only starts with the domain authoritative servers, and the Root Name servers.
Custom domain published blogs, to be consistently stable, can only rely on the DNS addresses being available on the local DNS servers, on a worldwide basis, 48 to 72 hours after the DNS addresses are added. This built in latency period has nothing to do with how custom domains are designed - and it's an issue over which Blogger and Google have no control.
Blogger provided the "Buy a domain" option, for custom domain purchases, 2008 - 2012, then briefly 2012 - 2013.
The previous "Buy a Domain" ended, in 2013 - now, we have "Google Domains".
Having permanently shut down "Buy a domain" in 2013, we now see the successor, Google Domains - which some customers call "Buy a domain - 2". Google Domains offers domain registration and DNS hosting, as a yearly package, with various popular features such as anonymous registration - all Google in house services, this time.
For all of the advantages of providing the domain purchase, registration, and setup, that just gets the domain paid for, registered with the Root Servers, and setup with the right DNS addresses. That does not get the DNS addresses, that point each domain to Google and to each Blogger blog, cached on the local DNS servers that we all use, as we surf the Internet.
The worldwide DNS infrastructure is immense, compared to Google.
The DNS infrastructure is hundreds to thousands the size of the Google DNS infrastructure, Think of how many ISPs there are, worldwide. Any ISP of any size is going to provide its customers with DNS service - if only to provide addresses for its own non DNS servers.
Google will never dominate the worldwide DNS infrastructure. Even domains which they sell, register, and setup have to be propagated worldwide, to every local DNS server - and there is the latency, that they do not control.
Allow 48 to 72 hours, for the domain to be visible, everywhere.
We still need a "Transition" period, to allow for every ISP, worldwide.
And the Blogger custom domain Transition period is needed, to allow for that latency.
And anybody who remembers Transition under "Buy a domain" should remember the mysterious gadget update period, that did not happen until after Transition was complete.
If you register a domain, using Google Domains - then use the Blogger wizard "Setup a Google Domains URL for your blog" to publish your blog to your new domain, you should get a dashboard reminder
Your blog is in transition
While that reminder is visible, your domain is being propagated, to the worldwide DNS infrastructure.
While you see the reminder - or even if you don't see the reminder - spend your time reading, and preparing for the migration process - that starts after Transition ends. You want your domain to work properly? Be patient.