What about working with a color transparancy, where the info you want to be shown on the hit is on the page but masked by the text being the same color as the background. The info that is part of the blog is text color different then the background and so its displayed. The text say black on black background wont show on your page to the eye, but is visible in the search.
Knowing how many characters are displayed of the blog you then know how many characters are required to hide the blog info and just show the custom description subtitle that you want.
Just an idea of a low tech way to do this maybe.