First of all you might want to relay the events. There is something nice about seeing everything explicitly in your main OF class. Simply create another class with the same blob callbacks and call them from the main class callbacks.
If you want to have multiple listeners you would have to hack this yourself at the moment. This should be fairly straight forward. Simply go to the cvBlobTracker files and change
ofCvBlobListener* listener;
to
vector<ofCvBlobListener*> listeners;
and also all the places where listener is used accordingly.
We should probably add this to the next iteration of the blobtracker anyways. At the same time I am also hoping there will be a general way in OF to register callbacks and listeners. There have already been some good ideas floating around.