A person who writes a blog is commonly referred to as a "blogger." Bloggers create content on various topics, sharing their insights, experiences, and expertise. They may write for personal enjoyment, to inform or entertain others, or as part of a professional endeavor. The medium allows for diverse styles and subjects, catering to different audiences.
A person that writes for another person's blog under their own name is called a guest blogger. A writer that writes the blogs as the blog owner is called a ghostwriter.
A blogger is a person who runs and/or writes a blog (weblog).
Usually someone who writes on a blog that isn't theirs is called a contributor or a guest blogger.
A person who studies or writes history is called a historian.
A person who writes with their left hand is called left-handed or sinistral.
A person who writes laws is called a legislator or a lawmaker.
Testator.
The person who writes a check is called the payor. Or in the case of my brother, a sucker, when he writes another check for his girlfriend.
An author.
a publisist a bigraphy
composer
a playwright