You may or may not want to do this. This is set on default when you make a blog. Basically, it is where there are those colored words and you have to type them in the box. They have it just to check that computers aren't commenting, but it may prevent commenters from commenting because of the extra hassle. So its your choice if you want to do this or not.
1. Sign in to Blogger.
2. Go to your dashboard.
3. On your blog, click "Settings > Comments."
4. Scroll down to where it says, "Show word verification on comments?"
5. Click "no" and you're done!