In my last degree, I completed a course where I created a blog post to accompany each assignment. You can see mine from #TECH6363 on this blog. Now I am teaching at the same institution and finally have the opportunity to teach a course similarly structured. I received a good question from a student on the amount of submission content in a blog post. It has a real-life answer and an academic answer. Much of that is answered by the public nature of the original content and your anticipated interaction with a program like TurnItIn.
When you submit to an academic institution, you can repost it as your work to your blog. Because, especially online, academic institutions have such an interest in avoiding plagiarism, it would be well worth your time to check in with every professor to ensure they are fine with this practice. While, as a professor, I am fine with it being word-for-word, good blog posts will likely be drastically shorter than a full paper. Please make certain to truncate your content by only posting about one of the topics (and obviously, rewrite it to make sense in the shorter format).
In real life, the best blog posts I did during my last degree came from discussion posts amongst my peers. While I didn’t have to give attribution, I did for some things. For instance, in this post, I mentioned a video that a professor had created because it was a source of a new idea/perspective for me. Even though I knew my readers wouldn’t be able to see it, I felt I needed to attribute that new thought to the source. These types of actions help you create authority with readers.
When you have published content online (in my case, 10+ years before I entered my last degree), the best thing to do in courses where you anticipate content overlap is to reach out to the professor early in the course and let them know about your blog and ask for their preference on citation/referencing. It is the order of publication here that often dictates the procedure. If something is already posted on your blog, you would need to cite yourself. Treat it like any other blog post course. It feels odd to cite yourself initially, but you get used to it.
Because of online institutions’ special interest in avoiding plagiarism in academic work, most higher education (and some K12) institutions have plagiarism checkers. When you submit, an instructor sees a similarity checker with a score assessing how similar your work is to other work available to the plagiarism checker. The database of what the plagiarism checker checks against for similarity include all member institution submissions (previous schools, previous courses, etc.) and anything published on the Internet (maybe your blog).
If anything has already been submitted via a service like TurnItIn (TII) or published on the Internet, I advise you to cite it.