You need to think about what you are trying to achieve when storing explosives. I would suggest there are three, not necessary compatible objectives:
1. Safety. Essentially you do not want the explosives you are keeping to become a hazard, or at least to keep the hazard to acceptable levels. You have two issues to resolve here.. Prevention, i.e. to stop it happening, and Mitigation, to minimise the outcome if something does happen. Fire is your main issue with storing Black Powder. You therefore want to keep it somewhere where it is unlikely to become involved in a fire. Black powder explosions mainly spread through dust. Keep containers covered and be careful when handling it that dust does not come in contact with flame. If the powder does ignite, then you want to make sure the quantity involved is small and the fire does not spread immediately to the rest of your stash. Segregation is the solution here.. small quantities spread around, so that if some goes off you do not lose the lot.
2. Security. Gunpowder is an explosive and can be used for making criminal devices. It therefore needs to be kept out of the hands of those who should not access it. Police and other security oriented authorities seem to like folk to keep gunpowder in steel safes and other secure storage containers. I consider BP in a steel box is a BOMB! Always remember that blast diminished asto the square root of the distance - double the distance, quarter the effect! Fragments carry the energy of an explosion considerable distances. Ammunition bunkers as used by the military are designed to stop fragments coming in to the store, not out. It is impossible to contain anything but a tiny explosion in a store..!
3. Storage. At the end of the day, you want your powder to keep in good condition. Black powder is a remarkably stable material provided it is kept dry and not moved about too much. I have examined BP removed from shell from the 1800s that is as good as the day it was made. BP does not suffer from temperature cycling in the same way as Nitro powders, although storing at a constant temperature will be better for the container. The secret is to keep it dry and sealed.
Recently in the UK the authorities have mandated that Black Powder be kept in 1lb plastic containers in a segmented plywood wooden box. The box has to have plywood dividers between each plastic container which come up over the top of the container. The lid has to be secured with an intumescent strip which will seal the box when exposed to heat. The aim is to delay the effects of a fire, and prevent the whole contents exploding at once. The box is designed not to produce fragments outside a limited area..