Firstly, it cant decrease your actual cost, any bulk deal mean the cost of per unit will cut, this is nothing to do with darkpool.
And how it works? Each "dadrkpool" is different effect, which is a few bits and pieces with LakeBTC's darkpool I found on the Internet with gg
http://cointelegraph.com/news/112112/a-ban-should-target-risky-products-and-foul-players-not-bitcoin-itself-lakebtc-ceo-thomas-xieCT: You have recently rolled out dark pool trading. Could you explain what this tool does and how it benefits users?
TX: From time to time we have users who come in and want to either buy or sell in large amounts, say 200 BTC. If you simply place an order like that it will obviously drive price up and down a lot. Besides, a public order-book is available to arbitragers, pro traders and predators. With advanced trading algorithms and strategies, they may take advantages of the information and affect trading price quickly. One way around this is hide your intention by splitting it to a number of smaller orders and spend hours or even days to spit out all these tiny orders.
Obviously not everybody can afford the trouble and time. Another method is dark pool. Note that our version of dark pool is not identical to the dark pools run by investment banks in equity market. Ours is more like invisible orders, which get executed with all other orders together. So dark pool orders still get matched with everybody else, but in a stealth way. You hide your intention and hopefully the market price will be less volatile and every investor, big or small, is happy.
http://www.coindesk.com/bitcoin-brokers-trade-millions-without-exchange/While many exchanges, including the current top exchange by volume, Bitstamp, offer stop-loss orders which can help mitigate slippage, one exchange has gone further. LakeBTC has released a ‘hidden’ orderbook feature that it calls its “darkpool”, designed to protect investors against ‘financial predators’ waiting to exploit the price distortions created by big trades.
LakeBTC’s darkpool works by hiding big trades of 50 BTC and up from the public order book. Although the rest of the market can’t see the trade, the exchange continues trying to fill it with outstanding orders on the market at the time, the firm’s communications director Lisa Li said,