Posts are being posted in two categories instead of 1

If you mean that the post shows up on the blog index and in the category index then that is how it is supposed to work.

Based on your statement that you “want [your] graphic design and [your] blog to be separate” then create a custom post type for your graphic design. Copy the “Books” post type from the Codex, place it in a plugin file, and edit the first parameter of register_post_type and the $labels values.

You will get a new menu similar to “Posts” and “Pages” for your “Graphic Design” and it will have its own archive page. It will be completely separate from your blog posts.