For those who have the time and are really interested in what batf says about its licensed black powder dealers storage, follow this link.
http://www.ecfr.gov/cgi-bin/text-idx?SID=b5b9b9cd93bd94a464721522837eae34&mc=true&node=sp27.3.555.k&rgn=div6
In it you will find several Types of storage.
Black powder is to be stored in a Type 1, Type 2 or Type 4 container.
Another link that takes you to section K.
This lists all of the various types of containers.
http://www.ecfr.gov/cgi-bin/text-idx?SID=b5b9b9cd93bd94a464721522837eae34&mc=true&node=sp27.3.555.k&rgn=div6
Just between you, me and the gatepost, most of these don't seem to have a "safety blowout" making them IMO, a bomb waiting to happen.
As for storing BP in an old refrigerator, I've heard some suggest this might keep the powder cool in the event of a fire.
If the fridge is one made back in the 1950's with fiberglass insulation it probably would work pretty good.
If it is a modern fridge with Styrofoam or other expanded plastic/rubber insulation, a good fire will melt the insulation and the powder will get hot.
IMO, a thin steel walled tool box or a box made out of thin plywood is probably about as safe as anything.
If the powder ignites, it won't have to build up very much pressure before the walls blow out.
It also can have the provision for a lock to keep idle hands out of it. :grin: