These are 3 ways to change fonts on wordpress.com blogs.
(1) Changing font families, colors and size in text in posts or pages either post by post or page by page by coding into the HTML (Text) editor. http://en.support.wordpress.com/editors/font-size-color/
(2) Changing font colors in text in posts or pages either post by post or page by page using the #4 icon (Select text color – change the text color) in Row 2 the Visual editor. http://en.support.wordpress.com/visual-editor/#row-2
(3) Changing all font families, colors and sizes throughout the whole blog by purchasing an annually renewable Custom Design upgrade and using Typekit Fonts with Staff support. http://en.support.wordpress.com/custom-design/
Note: A few themes do provide for changing font color in the Blog Title and/or optional Tagline and/or links. In the majority of themes font changes to font families, colors and size in the blog title, tagline, menu, post titles, page titles, categories and tags, widget headings, comments, and links cannot be done without CSS editing.
Also, I named the blog something and now want to change the name. How do I do that? I've changed the title but want to change the actual blog name.
Blog Names also known as Site Titles and/or optional Taglines can be changed at any time here > General Settings
However, if you want to change a Blog Address ie. the URL starting with http:// that's different. Assuming the .WordPress.com blog address you want is available, you can change it by using these instructions, > changing a blog address > http://en.support.wordpress.com/changing-blog-address/
Or if what you want to do is purchase your own domain and then it does not matter what the underlying .wordpress.com blg URL is > http://en.support.wordpress.com/domains/
edited by Tess to remove timethief from spam filter