As was stated earlier, a lot of it depends where you live. With me being in Louisiana I've kept the snakes in tubs with Aspen, with holes drilled and a water bowl and have never had any shed issues. I just make sure the water bowl retains water, and if I noticed one going into shed, I misted a few sprays, however I didn't have an issue without misting either. Now in a aquarium, I couldn't tell you as I would never put a BP in one.
Cypress Mulch I used when I had my Yellow Anaconda as it held good humidity, I was picky about what cypress mulch I used though. I at one point opened a bag of forest floor and they had glass in it, I only noticed because I always went through the substrate prior to putting it in my snakes tank for exactly this reason. I sliced my finger on it and I took pictures of it and spoke with Zoo Med. They ended up sending me like 3 free bags, though I wasn't trying to get free bags, I simply wanted them to do a better job of filtering stuff or whatever process they use. I think after that I started getting a certain kind from Lowes, and it was just like forest floor just a lot cheaper. This was years ago though so I don't remember what I used or even if they still make it.
CoCo Husk as Deborah mentioned, is a great alternative. It holds humidity well, and won't mold over and it's not sharp like cypress mulch. I use this on anything requiring higher humidity. You can get the little bricks of CoCo Husk off Amazon for like $3.