Ah, are you saying that the blog is hosted elsewhere, rather than under your account? If so, it might be difficult to grab the latest post. Possibly you could (in PHP) open a fixed URL of the blog's latest posting (without all the surrounding page), and read it in something like
// get first three lines of latest blog posting
$fp = fopen("http://www.blogger.com/posts.php?which=latest", "r");
$count = 0;
$first_lines = array();
while (!feof($fp) && $count++ <3) {
$first_lines[] = fgets($fp, 1024);
}
fclose($fp);
If you have to get the entire page, it's a bit more work to discard all the stuff until you can get to the posting itself.
Of course, if the blog (and its database) are under your account, you should be able to get to it directly, either through some sort of blog interface call, or directly reading the database.
You'll have to tell us what blogware you're using, and whether it's all under your account or the blog is off somewhere else. At that point, someone familiar with that particular software can tell you how to proceed.