In the United States, it is legal to write a blog about someone, as long as you do not reveal their identity or personal information. However, there are a few caveats:
First, you must have the permission of the person you are writing about. If they do not want their name or personal information published, you will need to refrain from publishing any information that would identify them.
Second, you must make sure that the content of your blog is fair and objective. If you are writing about someone with a negative bias or with the intent to harm them, your blog may be considered libelous and illegal.
Finally, remember that blogs are public records. If you receive a cease and desist letter from the person you are writing about, be prepared to provide proof that your blog is legal and that you have followed all of the guidelines established by law.