To clarify a little, there are different types of water when it comes to river-born fish. Clearwater, Whitewater, and Blackwater. Many P's come from the Blackwaters of the Amazon. Blackwater is "jungle water" basically.
(Sciene)
To read off the product that I have, Blackwater Extract contains trace elements, vitamins, and valuable extract of peat. It creates and enviroment similar to part of the Amazon River and other natural, blackwater biotopes. Blackwater extract promotes fish activity levels. When added to the aquarium, a light yellow color will appear.
(/Science)
I've been using it in my aquarium with my P's for about a month now. It know the Extract isn't the single cause for the appearance of my fish, but I'm sure it helps, because the P's are looking good.
Anyways, I picked up an 8OZ bottle of it for about $6.50. If I use it in the right doses at the right times, this bottle should last for a little under 5 months given my tank size.
All in all for the look of the tank, it doesn't do much. You can see a slight tint, as the bottle described, but that's about it. I apologize for those I might have put under the impression that the extract darkens the water completely.