It seems to me the predominant use of Bitcoin today is holding.
That's probably true.
Then I guess it's a toss up between general black market activities and speculative trading.
That's probably incorrect. The last study I saw put Bitpay (maybe also including other smaller processors) and black market activities in rough parity, speculation being larger than both.
As for the rest of your comments about secret trading desks and such, well maybe. It's unproven and largely unprovable so I guess I'll just wait and see how and if that develop into something significant enough to become obvious, because it currently isn't.
Maybe I wasn't clear enough but by black market activities I included capital control evasion and other similar use cases.
Obviously I cannot prove it but I am confident much more Bitcoin moves OTC than on exchanges. I would also venture to say that except those trading desks that are under legislation in the US it is likely the rest of them do have any interest in "blacklists".