Like the thing it acts against, [it is a Firefox plugin][link]. Were Firesheep a serious malicious tool, the race between BlackSheep countermeasures and Firesheep countercountermeasures would be endless, and Firesheep would always “win”. You can’t develop countermeasures until an attack has been successful. This was and is the message of Firesheep to begin with: The only solution to this kind of attack is to use HTTPS.
[link]: http://www.net-security.org/secworld.php?id=10118 “Firesheep countermeasure tool BlackSheep”