http://pixelclouds.com/2010/04/03/design-blogs-with-amazing-vector-backgrounds/ Are these available in wordpress? Also how do people go about creating their own backgrounds and themes for their blogs?
Well, It doesnt depend on which platform that you are using for your blog. Its just a design and will work out with any Blogging Platform including WP if you know how to go about editing design. You can create these backgrounds using software like Adobe Photoshop or GIMP. Using the same, you can create unique designs for your blog. What I mostly do is, select a template which i feel is good enough for the rest of my blog and then create a unique header or something of that sort and add it on to my blog so that it looks a little different than the template we use. If you need help regarding the same, let me know. Prateek, PDJSolutions.
You just need a comfortable level of understanding of the chosen blogging script in order to do it. Plus, as pdjsolutions said, you need GIMP or Adoble PS to do it.
If you have not noticed the examples you quoted (one of them here) are wordpress sites... refer wordpress theme development documentation and if you already have a wordpress theme, for which you have found out a perfect background images, just modify style.css to achieve the result