The site license explicitly allows this. Read the small print (bottom of the page), you have to give proper attribution. Post two links, one to the author's profile, another to his post. This goes a long way towards taking the sting out of him feeling you stole his answer. Of course, don't forget to mark his post helpful. It was.
Comments are a bit trickier since you can't link them, I've always seen the author's name being mentioned in the edit to be sufficient.
Arriving at the 'perfect' answer is the site's stated goal. Some of the highest rated questions and answers are blends.