I'm using the following CSS to do this here:

{ font-style: italic !important; text-align: left !important; }

…In conjunction with the [Center] BBcode.
(The “text-align: left” part is what keeps the text from being centered by the BBcode, BTW.)

I also tried using CSS to try to italicize text by using the [Quote] BBcode instead, but it doesn't seem to work…
(At least, not in blogs, anyway.)

Oh, and to use CSS in blogs you use the [Style] BBcode, BTW.