I actually find blogger quite a nice place to test out new ideas for websites
In the recent past I have tested and weeded out several websites, and within a month, was able to figure out wich ones would work, and wich wouldnt.
For the ones that work, I then create a new (proper) website, and post a message on the blog telling users that the site has moved, and I also redirect any links in older messages to the new site.
On top of that I summarize all the older posts (Pretty much the first paragraph, and repost it with a link to the new page in the new site (Read more type of thing)
And then I start emailing the guys that linked to me to change their links.
This approach does meen you have to basically do everyting twice (From an SEO point of view), but since you know the site will work, its worth doing it.
Ontop of this, once your site is not a blogspot site, it becomes easier to SEO it, so the traffic you get is generally higher than it would be on the blogger blog.
If I find that one of the failed blogspot url's might be usable for another related site, I keep it as is and forget about it until I have the time to work on the next idea.
If the site is completely useless, I delete it.
It might be a but hard for you do do this with your blog . because of all the posts, but it is well worth a try I think.