There is some correlation between flash and blast. Flash is caused by flammable but unburned gasses in the cloud that forms after muzzle exit. When those gasses mix with new oxygen in the atmosphere, they can spontaneously ignite in a near-explosive way, producing an additional shock wave. Powders with a flash suppressant help control this.
But the major cause of high muzzle blast is simply the release of high pressure, like popping a cork. At 10,000 psi or more.
