You mean you're planning to write a guest post for somebody else's blog? In which case, yes - your post should be original. Why would somebody want to accept a guest post which isn't original?
You might be able to use an idea from another blog as a starting point for your own original article, provided you give due credit to the original source.
e.g. "I read an interesting post by so-and-so the other day, saying blah, blah, blah. That got me thinking about the subject, and I think he's right/wrong/a genius/whatever. Now here are my original thoughts on that topic, with maybe the odd reference to points in that article."
But you would need to be very careful how you do that and ensure the bulk of your post is really your own, original work, and not just a spun version of the original article.