Sure they can. Just make sure that if you are using a blog post as a sample, it has a certain level of professionalism. If all you're doing writing a journal or diary and complaining about the world! then nobody wants to see that. It's probably not gonna get you any job. But if you're say a travel blog and you wrote 2500 words on traveling, then that's gonna be not only a great portfolio piece but it's also something that can help show off your personality.