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 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 purchasing an annually renewable custom design upgrade and doing the required CSS editing. http://wpbtips.wordpress.com/2009/03/24/color-customisable-themes/