It was described as flash powder. Similar to photoflash powder used in olden days to take flash photography of bomb damage. The material doesn’t burn/deflagrate; it always detonates. Its classification for military purposes is a high explosive unlike black powder and similar which burn on the surface.
So, 42lbs. of high explosives, not really fireworks though a great flash producer.
Way back when, I bought some flash powder and black powder and booby trapped an ashtray. When it didn’t trigger the way I thought it would have, I forgot about it until it went off about an hour later. The results of a small pack of flash powder bought at a magic shop and a dozen grains of black powder was enough to blow cig butts into the next room. That was an uncontained reaction, I am willing to bet that if I had done it in a soup can, metal shards would have flown.
Armchair-quarterbacking this, after a break from the news and the SDMB, but with a little practical experience on my side: no. I would not simply place a detonator in a package of black powder and hope for success. I would use a booster charge, which, normally would have been a bit of C-4. Detasheet might work in a pinch, but “C” is plentiful, and IMHO, a little easier to handle, cut, and shape.
Flash powder, too, is a little too “loose” of a powder, that I’d want to throw some more heat/shock/friction into the mix to make sure it is a quick, clean, and efficient detonation.
Tripler
I 
C-4.
I was wondering what happened to the Illudium Q36 I lost.
Well, okay, but at least it did go “kaboom.”