Hmm.... this is actually more well understood than you think....Smoke is the biproduct of the fuels it is burning, you just have to understand it's scale size to what you see and apply the same science.
The color of the smoke indicates the type and density of the fuels involved (such as your bubble hash), and if compared to a larger scale and involved larger building structures, then it would indicate to fire fighters hints as to what the fire might do next. Ok, let's not get off topic.
All materials burn, and depending on the intensity of heat to that material will give off different smoke indicators as to the types of products that are burning and it's intensity to burn, combined with the intensity for that product to remain combustible to it's vapors released. Resins of cannabis are generally considered oil based in nature, and the more concentrated (your bubble hash) the more intense heating will be needed to create a cleaner combustion process to sustain combustion. I would suggest that your method of heating your bubble hash, combined with the moisture content in it , takes more time on initial lite up, and you are getting a blacker, more sootier, smoke as a result of trying to light it to fresh, or with less heat than required, but once lit probably subsides somewhat to a lighter color as you drag and increase oxygen to the equation.
White smoke can often mean material is off-gassing moisture and water vapor, meaning the fire is just starting to consume material. White smoke can also indicate light and flashy fuels such as grass or twigs.
Thick,
black smoke indicates heavy fuels such as concentrated oils, that are not being fully consumed. At times, black smoke can be an indicator that a man made material is burning such as oil, tires, vehicles or a structure. As a general rule, the darker the smoke, the more toxic the gases , and volatile the fire is.
Grey smoke can indicate that the fire is slowing down and running out of materials to burn, like at the end of the burn of a pipe.
Here's a video of an example of when you see black smoke with hash burning, and though it probably does not represent completely with what you did or do, it does show puffs of black (sooty) smoke on the initial application of heat, until such time as a more complete combustion continues (whiter color of smoke), and as the fuel load dwindles then it will darken to a grey colored smoke. I know this is rather lengthy an explanation, but smoke color is more understood today in combustion to understand this answer.
Sorry for the long answer to maybe a more simple question you expected but it is my passion and career in understanding this subject matter, so I offer it as a science truth. I hope it somehow helped, and fixes your problem
