Now Robert gives a good answer below, which I would agree with, if you are just starting out...

From my perspective, building your blog here at WA, means you have your own business, so I would go for the paid domain myself.

Always best to start with a free SiteRubix.
This gives you the opportunity to work with and understand what you must do to create and build out a website.
If you like it then you can always buy your own domain name and transfer everything over to your own domain name.
If you don't like it you can always delete and start again.
Enjoy your Thursday.
Robert

Hi Parish - It is up to you. I went premium and bought a Domain with .com and did all the training offered step by step and everything is great in my online growth however you may prefer to build a site Rubix website
Either way, enjoy yourself
Vicki

Start with a free Domain just to see how you progress, if you are happy with the progression and want to keep moving forward, moving your free Domain over to a paid Domain is made very simple here at WA with the "Move" option, no content is lost