Burned in the open, gunpowder doesn't blast. It's a bit like a sparkler. Faster than a sparkler, but no "blast." The only way to make gunpowder, even smokeless powder, go "boom" is to contain it so pressure builds. Under the high pressure condition, it burns faaaaast! and goes boom.
Your explanation is generally right. Just a nitpick about low explosive "blast," and pointing out there is no blast at all, unless the low explosive is contained.
Yes, I shouldn’t have said “blast”, in referencing gunpowder in the open. My main point was that containment is necessary to get high-explosive-like blast from a low explosive, such as gunpowder.
When under pressure, it may burn faster — although the temperature build-up is also a factor in increasing the rate of burning. Even if it just continued to slowly fizzle away, like a sparkler — when it built up enough pressure to burst the containment vessel, there would be a blast.