What massive alchemical factory. they have a lab and mix up amounts as they go, making up a few kegs. Then for cheapsies, they start blowing stuff up that was supposed to be a good hard fight, trivializing encounters repeatedly, and start breaking the structure of the setting as suddenly there's a power grab on for exploding black powder.OP just said Black Powder, not alchemical black powder. Not that that makes any real difference - it still needs to be made from something and requires time and money to make. If they're building some kind of massive alchemical factory that attracts attention.
I really don't get it. What is supposed to happen here? The PCs are going to take out there enemies by cleverly smuggling barrels of black powder into their headquarters and lighting a fuse, while making sure they are out of range when they explode?
This is a ... bad thing?
It's a tool. It's not an "I win" button. It requires some degree of logistics and planning to make use of, and the world around them will quickly adjust if they keep pulling the same trick (the effects are not exactly all that subtle).
And there's been a LOT of conflict over what 'black powder' is, so no need to go high horsey here. Given it takes alchemy to make, it's definitely not purely gunpowder as we know it (which all comes in multiple grades, anyways)