it's not all dark pools. That's just as tough to hide, not get gamed, etc. They're using a combination of brokers, DMA, and using old relationships to try to keep things quiet. There will always be some form of leakage. Also, you'll see these guys ALWAYS looking fo rnew methods, technologies, etc - that's why buy side trading conferences are all about. No algo is going to mask it all.
Some tend to try to use brokers where there is huge, anonymous quant type flows. It's a good way to snake in some names before people realize what's going on. They have to mask huge orders anyway they can, because guess who the phone brokers call once they have a large order? Yes, the natural on the other side.
It's not just dark pools....