Reasons why a blog is generally not considered to be a credible source
Blogs are written by an author or a group of authors and generally cover a specific topic. Blogs, short for Web logs, started as simple online diaries with a specific structure, with new posts always appearing on top of the website. Blogging has become extremely popular and there seems to be a blog on every subject under the sun, so it is entirely possible that there is a blog out there relating to your research topic.
In general, blogs should not be considered a reliable scholarly source. While some of them are written by real experts and can be great sources for some topics, many are strongly opinionated and can lack the professionalism expected in a scholarly source. In some papers on contemporary or Internet-related topics, it might be appropriate to use a blog though, especially now that many famous politicians and media personalities are blogging.
As with any source, be sure to check the author's credentials, evaluate the blog for bias, and check how often it is updated. Blogs may often be sponsored or endorsed by other, sometimes larger organizations, and that information should be made available on the blog. Also, blogs can make money by selling ad space on their sites, so beware of excessive ads. Beware that there is no bar for entry and no quality control for blogs. While books and journal articles make it through several review and editing stages, blog authors can essentially publish anything without anyone fact-checking it.