In the United States, there are two types of copyright – statutory and common law. Statutory copyright is created by law, while common law copyright is created by court decisions.
Generally, statutory copyright lasts for the life of the author or 50 years after the death of the author, whichever comes first. Common law copyright lasts for the life of the author plus 50 years.
There is no one answer to this question since it depends on the specific situation. If you are writing a blog post for personal use, without any intention of selling or distributing it, then you likely do not need a copyright.
However, if you are writing a blog post with the intention of selling or distributing it, then you likely need a copyright. It is important to consult with an attorney to determine whether or not you need a copyright for your blog post.