Coco bark is fine to use, holds humidity well. For our BPs we just stick to newspaper now, as with many of our snakes - although it doesn't look as nice, it is cheap, easily obtainable, easy and quick to clean, and the snakes use the whole enclosure as one big hide because they can sneak under the paper!
Other substrates that can be used include the aspen bedding or the wood chips, although these materials aren't so good for the higher humidity. Moss could also be used, although don't allow the humidity to get too high, or you could use paper towels or carpet.