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.