Getting your bark to a fine powder can be quite difficult, I know this all too well because I collect and process my own. What I've found works best is, once you have it reasonably fine (I'm talking small wood chip consistency) is to soak the bark in a minimal amount of fresh water so that the fibres absorb the water, expanding and softening in the process. Next, freeze the wet bark, thaw it and allow to dry (you can use mild heat or a desiccant if you wish to accelerate the drying process). From here they should be much easier to powderise in your blender. Don't forget to keep any extra water the bark doesn't absorb and add it to your extraction, as it may contain small amounts of spice.
"Laws alone can not secure freedom of expression; in order that every man present his views without penalty there must be a spirit of tolerance in the entire population." -Albert Einstein
I'm not a big fan of SWIM. I mean, I've never met the guy, but any time I hear about him, he's doing something sketchy.