To answer your question you must learn a very important difference: The difference between a software and what you create with a software.
The GPL is a copyright license. Copyright can not limit what you produce with a copyrighted software, it can only limit the use of the software (like copying it).
That's important to know, as humans need to be able to interact with a work (the software), even the interface you're using does not fall under copyright. It's only the software that is.
What I want to basically say is that the GPL can not apply to a programs input and output if it is not related to the work itself. The in- and output in your case is your blog-post. It's a work on it's own under copyright (let's assume what you writing falls under copyright) and as you're the author of that work, you can decide freely on your own about the licensing of your texts.
That's your right no software license based on copyright can keep away from you, that's independent to the GPL.
The GPL even clearly states that in it's licensing terms, for example in those GPL licensing terms shipping with Wordpress:
The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program).
As your blog post is not a work based on wordpress (in the meaning of work and copyright, that's meant legally here), you're free to do whatever you want.
Please see as well the GPL FAQ:
The GPL only ensures that the software needs to stay free for all of it's users - current and future ones. It's using the copyright to achieve that. And it grants you the freedom to use the software for any purpose, including producing texts not under GPL ;)