Wild BBGs are found in both fresh and brackish water. At least some species occur in "blackwater" streams where the water is soft and acidic, but since hobbyists can't identify them to species level, there's no way to make sure you have species suitable for a softwater aquarium. On the other hand, all BBGs will do well in either moderately hard, neutral to basic freshwater or brackish water. Choose whichever suits the other fish in your tank. They may well be *easier* to keep in brackish water insofar as adapting new stock goes, and avoiding problems with acidification and pH changes, but a (moderately) expert fishkeeper will be able to keep them just fine in a freshwater tank.
Most BBGs die not because of water chemistry issues but because of lack of food. They are notoriously tricky to feed. Frank Schaefer argues that the advantage of brackish water is you can keep them surrounded with live brine shrimp, allowing them to eat as often as they want. Once tamed though, you can usually get them to stuff themselves on frozen bloodworms and chopped shrimp and so on.
Cheers, Neale