Would citations in blogs, public materials, arXiv and magazines increase citation count for the author?
Scopus counts only peer-reviewed articles.

What Google scholar counts is less clearly defined. Intuitively, if something looks like an academic source, then its citations are counted. This means that arXiv is definitely in. I've seen slides for talks hosted on academic webpages being counted, but not in a very consistent manner. I would suspect that most blogs don't count. Mathoverflow does not count.