I would recommend picking a minimalist, clean and clear blog design, with a white background and black text. As boring as that may sound this ensures that the content you upload to your blog is readable and clear, after all, you wouldn’t want to spend a lot of time writing a post just for nobody be able to see it.
If you choose to buy a design for your blog, it’s important to really budget and plan. Don’t go wasting a lot of money on a blog design if you think that two months down the line there’s a chance you will have stopped blogging. Maybe even leave it a little while before actually buying a design, so you can blog for a little and see if you like it. Once you know you’re serious about blogging, that’s the time when it’s safe to allow yourself to spend a bit more money on a layout to make your blog look more professional.