There was someone who needed 200,000 Bitcoins around March of last year. No one really took them seriously till it became evident that the price was exponentially increasing. I'm sure someone has a link to that original forum thread. At the time I think the price was between one or two dollars. MtGox still did darkpools at the time.

From IRC chat logs and the like, it appeared as if they started purchasing up till the $4 range and then it became evident they started selling to see if the asks were being changed accordingly, when they realized their buys at the ask were only feeding more and more accumulation (I think the IRC log indicated they sold ~20,000 Bitcoin and the buys were eating it right up) they continued the ascent.

That said, there was also a ~200,000 Bitcoin trade around the same time that was NOT done on an exchange, so it could be that the price increases had nothing to do with this mystery person. MtGox dealt with them directly and discreetly, if the IRC logs are to be believed.

Only a few people seem to remember this whole incident for some reason, but it seemed as if someone *really* wanted Bitcoin at the time and whoever it was, they got it.

If you really do need to purchase a large amount of BTC, you'll notice from mtgoxlive.com that you could drop $250,000 on asks on MtGox right now and the price would only move up to $7.04. My guess is that you don't really need a darkpool, you just need to decide if you trust MtGox to handle your money or not.