Hi Ben,
I am not sure, but i think you will have to do more than just specify a new font in css. I think the font has to be called in the theme xml.
In any case, generally in Blogger, "body" is the selector for all the text, while ".post-body" and ".post-text" control more specific blocks of text.
I do not see how this can work unless you also include a call to the font in the theme, though you could give it a try. If you google around you can find various instructions about how to do this.
If you do crack open your xml,I strongly suggest backing up your theme first. Can't do any harm, could be a life saver.