It's a deeply satisfying way to defeat an attacker: give him a taste of his own medicine, and the BlackSheep add-on attempts to do just that. This add-on listens for Firesheep's patterns on the network and warns a user that they are being watched. BlackSheep drops juicy bait for Firesheep (fake session cookies wrapped in legitimate-looking requests to sites) and waits for Firesheep to pick them up. When a Firesheep attacker tries to exploit these fake insecure sessions, BlackSheep picks up on it and alerts its user.
While there are some Firesheep users that will set off this alarm, the problem is a bit more serious. Many attackers have probably looked deeper into the fundamental flaw and also want to protect themselves from such an attack. These folks will be using a security-forcing add-on like Force-TLS, STS-UI on Firefox 4, HTTPS Everywhere or NoScript. All of the protected bad guy's connections to vulnerable sites will be secure, including the sessions that have been copied from other users on the network.
The upshot is that while BlackSheep may see the attacker connecting to vulnerable sites, it won't always know when the bait was taken. For many serious Firesheep-like attacks, BlackSheep will remain quiet.
There are many Firesheep users who are just playing around with the new, hot hacking tool, and so BlackSheep will cry out some of the time; it can't however be relied on to detect most Firesheep attacks. While I like the idea of an alarm like BlackSheep, solving the underlying problem (and stopping serious attackers that are more of a cause for concern) is a bit more complex and requires a different approach. Ultimately, we need to secure wifi networks, protect ourselves with HTTPS-forcing technologies, and ask sites to protect their users by using HTTPS for the whole session.