If your blog is marked as a spam blog you can submit a request to Google to get it humanly reviewed and cleared.
After blogging for nearly two months and a handful of posts, I had one of my blogs marked as a spam blog. All of a sudden I had to type in a verification word for every new post or edit I do. I thought it must be an additional step to ensure that it's a human indeed who's posting. One day I casually clicked on the red colored help icon (?) next to the word verification and then only I realized that my blog is marked as a spam blog.
The help page said that I can submit a request to get my blog manually reviewed by Google to get the spam blog status cleared. So I did and within two days I got a mail from Blogger Support that my blog is reviewed and cleared. So I'm back to posts without word verifications. :-)
I just can't think of a reason as to why my blog got marked as a spam blog. I did not post frequently and all my content was original and not just copy and paste work. Blogger Help states that blogs that perform link spamming get marked as spam, again something that my blog didn't do. Nevertheless the spam detection algorithm thought that it's a spam blog and I think Google should improve on it. However, the Blogger Support was quick to respond to my request and clear my blog.
If you, too, have to enter a word verification to post to a valid blog, go ahead and get your blog cleared.