Though I've not actually been there, it's my understanding that there is very little true aquatic vegetation in the blackwater areas. The combination of high concentrations of tannins and the very low pH (interrelated) result in an inhospitable environment for underwater plants. The banks would be quite grown over, though — probably with multiple species of Echinodorus (swordplant). If I were doing a blackwater tank, I'd try Eleocharis (hairgrass), Myriophyllum (foxtail), Heteranthera, or Cabomba. And of course whatever Echinodorus seem appropriate to the size of the tank. I'd also try to keep the pH from going below 5.8 or so, and make sure to have enough light to compensate for the reduced transmission caused by the water's tint.