Cerner has the most comprehensive modern integrated suite of Medical software systems. If you go to them they will have the answer, or probably know where you can go for answer.
A "Blue Box AI" system is available for download here (Power PC Macs only) but it is not billed as an expert system nor, apparently, does this particular Blue Box AI system have anything to do with diagnosing/treating depression.
Might you possibly be confusing this with a blue-light treatment for S.A.D (Seasonal Affective Disorder)?
Would you elaborate a bit about what you're looking for? It might help us considerably if you tell us what you know of this system and where/what you heard about it.
I have programmed a simple expert system using Prolog to identify the pacing modality in Cardiac Pacemakers. Prolog is used in AI Artificial Intelligence. Use the DSM-4 for your database it worked very well!
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Regards, Maveric Manic - 'Knowledge is Power and Wisdom is knowing how to use it'