For this Blogspot blog I have I want to use a domain name. But my problem is will I loose all traffic and serp or Google put a 301 redirect to the domain name ? .
As far as y experience with these things is concerned, there is NO WAY to put 301 redirect with blogspot domain. Al you can do is that you can use a meta tag that will redirect your blogspot blog to your new page.YOu may retain some traffic but that is not very effective for preserving the link juice. An I am pretty sure about what I am saying. I recently did a 301 redirect from www.amoledcreen.net to www.irnovo.com both are on blogger. Had it been a blogspot domain, it was impossible to do a 301 redirect.
Blogger automatically redirects yoursite.blogspot.com pages to yoursite.com, once you buy the domain through them. So, traffic not much a worry in that case
only the homepage is redirected to the new hompage and its not a 301 redirect so link juice will be lost if any.
It is made automacally for Google, he makes a 301 redirect. Even there's an option to redirect domain.com to www.domain.com