The amount or lack of flame from the muzzle does not necessarily provide any indication of load performance with either black or smokeless. The powder provides its own oxidizer source in a volume sufficient enough to obtain an efficient burn of the particular powder type.
This topic comes up quite a bit on the smokeless side and the confusion comes from trying to compare a contained burn with an uncontained burn. No matter if it's a gun barrel or a blast hole in rock, there is some type of charge containment. When the fuel is ignited, it becomes a self-contained chemical chain reaction where the oxidizer and fuel mixture rapidly decompose until all of the oxidizer is consumed or the containment is breeched.
In the case of a properly operating gun barrel, the containment is breeched when the projectile clears the muzzle. No fuel (power) provides 100% efficiency, no matter how the fuel is consumed in a containment, there will always be some degree of inefficiency. For example, only about 50% of the hydrocarbons in gasoline are actually burned in a common internal combustion engine; if the exhaust pressure and temperature is sufficiently increased, one would see the majority of unburned fuel oxidize as open flame once it cleared the containment of the exhaust pipe and was allowed to combine with the free oxygen in the ambient air.
Guns are no different no matter what type of powder you're running them on. On the smokeless side, certain powders like Alliant Blue Dot and IMR 4227 can work efficiently with a given load & barrel length yet still produce quite an impressive muzzle flash. The discussion came up about a particular 12ga cartridge load using Blue Dot where the fellow was getting a huge muzzle flash from his 20" barrel yet the same loads produce about the same amount of muzzle flash from a 36" barrel. Reason being, the muzzle flash is generated not only by the percentage of un-consumed fuel that is present in every load but more so from the volatile gases remaining in the bore after the majority of fuel & oxidizer has been consumed within the containment. The operating pressure of the load also plays a roll in the amount of muzzle flash one sees with a given load combination. Loads such as the 12ga running on Blue Dot operate at less than half that of what a similar charge of Blue Dot produces in a rifle; the higher operating pressure of the rifle produces far less muzzle flash yet there is no change in the efficiency of the load. The efficiency is determined by how much power charge is converted to useful mechanical energy.
Just where a typical piston type internal combustion engine produces less than 20% efficiency at converting the fuel to usable mechanical energy, any powder charge in a gun & load combination is going to have only "X" amount of mechanical efficiency. Thus, no matter what the gun/load combination, a certain percentage of the fuel is going to be wasted and the tipping point becomes the trade-off between how much is wasted in relation to the obtainable mechanical energy output. In other words, if one sees a 100fps gain per every 10gr of powder increase then at some point the 10gr powder increase only produces a 30fps gain, the tipping point of efficiency has been exceeded.
The major difference is between a contained and uncontained or semi-contained combustion. For instance a firecracker or stick of dynamite; when they are detonated in free air, one sees a single flash and it's all over because whatever amount of the fuel & oxidizer mixture as well as secondary flammable gases produced by the initial burn are allowed to readily combine with the ambient oxygen in the free air surround it. When the fuel/oxidizer burn is contained such as in a gun barrel, the muzzle flash is composed primarily of waste materials.
This pic is a little different than what we're dealing with but it's a good example. Note the lack of flame at the muzzle but the considerable amount of flame that is visible several feet beyond it.
Here is another example where the most intense visible flame is 80+ feet beyond the muzzle indicating the flammable materials ejected from the muzzle lacked an oxidizer. The same is seen when blasting bore holes, the flame is never seen at the surface but rather at some distance away from it.
