Now what happened in the past when you change your blog address was that it automatically releases the name for registering for you + others. The spam dudes are pretty darn good these days and if you aren't that careful in the past they could sometimes find your old blog address within minutes (especially if it was a really old one.. with links pointing towards it.. , etc. Wait why u wanna change its address then!.. oh it sucked OK).
So let's say after changing the blog address you wanted to get it back but you felt really tired and went to bed thinking "I'm gonna get that old sucker tomorrow.. no one knows it is available for registering but only me :(". Well in the past it could have been a nightmare, waking up in the morning and trying to get your old URL back.
But I don't really know when but I changed this blog's address recently and Google automatically blocked the old URL and it is now only available for me to register anytime I like (well to be honest I don't know if there is a timeframe for this).
So even if one of your readers came through the old address they'd see a message called
Sorry, the blog at thenamethatsucked.blogspot.com has been removed. This address is not available for new blogs.Which was in the past gave something like ... "the blog thenamethatsucked.blogspot.com cannot be found but is available for registering!" yikes for you hurray for your reader. So this is pretty cool actually.. I feel safe already ;-).
So, thank you Google Blogger team.