Outside of speculation, what's likely the largest use of Bitcoin? Drugs. A billion dollar industry.
Drug dealers use Bitcoin for their anonymity. Their profit is off the illegal product, whether the medium of exchange appreciates or depreciates is probably either secondary or of little importance to them. The stability, however, is of importance. Like any other vendor, being able to keep price relatively stable allows them to forecast profits.
An industry with the money, the need, and the means to keep Bitcoin stable. And lo and behold, Bitcoin price has become very stable (admittedly over the short term). I'll admit even as I write this myself it has a little "tin foil hat" ring to it. But I don't think it's out of the realm of possibility that we're seeing a coordinated effort by the drug market to keep Bitcoin prices stable. They're the only industry right now that really uses Bitcoin to a great degree and has the capital to move the market as they see fit. If all this is true, I would figure this flatline trend will continue for quite some time.
Do you really think (most) people are trading on Mt. Gox just to score some dope?
If you want to talk purely "black market" I would say the largest use of Bitcoin is (or soon will be) gambling. It allows circumvention of the online gaming laws that have been popping up around the world.