BPs don't require super high humidity but how you want to apply the sapghnum moss depends on how quickly the cage loses humidity and how much humidity you need to add. ie Are you using a tank with a mesh top or any enclosure with a solid top? Solid tops hold humidity better.
Also, what is the ambient humidity of the room the BP is kept in?
Here where I live in Florida the ambient room humidity is already at minimum 60%+ thereby I use a mesh top and keeping the eco-earth semi-moist is more than enough humidity (I frequently have to let my BP's tank 'air out' just because it will regularly hit 80%+ humidity if I restrict the circulation much at all). In places with a lower ambient humidity, a screen mesh top will cause the humidity of the tank to dissipate very quickly and will cause you to loose whatever humidity you add very quickly. Thereby I think it would be pointless to try and add moss to a large portion of the tank with a mesh top as it will just dry out very quickly. If you had a solid/covered top and needed to add a lot of humidity, sure, add sphagnum all over the cage - just make sure its only damp and not sopping wet or you are courting scale rot.
I'd recommend doing a moist hide. If your BP is small, a cup or (what I like to use) a plastic or styrofoam box that mushrooms from the store come in. They're just a nice size for smaller snakes I find. And tall enough to stuff a good handful of moss in so the snake has plenty to bury in.
For a bigger snake, a plastic shoebox, cat litter box or other plastic container of the right size with a hole cut in it for the snake to get in and out of filled with moss is what I like.
Having the moss inside the hide with a non-permeable cover (the hide itself) prevents the moss from evaporating as quickly, creating an area where the snake can go to obtain more humidity as desired.
You can see on of the hide boxes in my baby BRBs tub here. Its a mushroom carton with a hole cut out of one end filled with sphagnum. And little holes poked in the top so I could stick some silk flowers on top just because I like my enclosures to be pretty.