As long as it is on-topic, it's fair game, because if the blog entry would not look out of place if posted in the forum, it's ok to put a short and tastful note of its existence. If done correctly, you are strengthening the community by making pertinent information available by sharing high-value links.

Three exceptions.

Exception one: you shouldn't explicitly charge or ask for money on the blog entry (modicum amount of ads are ok, though, to help defray bandwidth expenses).

Exception two: if there is a more appropriate venue, such as a Planet <X/Lisp/Ruby/etc.> that acts as a syndicate for blogs and covers your area of interest, then the blog syndicate should be used instead since most people understand that that's the place to go if they want to read blog entries.

Exception three: don't bother spamming your lesser blog entries that hold little new value.