One piece of advice that I often give people for their resumes is to have two other people review their work. One in their area of expertise, and one outside of it. The same thing should happen for your blog entries.
As you blog, even if you are doing it privately as a portfolio and not as a public expression of your knowledge, you ought to find two people that you trust and ask them to review some posts. Let them read your work and give you opinions about the way you write, the impression you make, etc. If you can find more than two people, send them different posts, and get feedback.
Writing is a skill and it takes practice, but to improve, it also takes feedback.
A blog is a great way to show off your knowledge, and a way to improve your communication skills, but only if you work on it and find a way to clearly express your thoughts. If you end up building a blog that it poorly written, hard to read, and does not convey your knowledge effectively, then it could do more harm than good.