Hi Steve,
The short answer is "don't worry, its normal", but that would be a
really boring answer to a good question.
I don't think this has anything to do with your cleaning procedures.
It is likely a chemical change in the flux itself.
I am making the probably true assumption that you are not using a
carburizing flame.
If you were, the wet flux would pick up carbon soot from the torch and
discolor, leaving a glossy, but blackened flux.
This color would go away fairly quickly as the carbon is burnt off.
From the picture it does not appear that you are overheating the flux,
burning a bit, and leaving a mix of burnt and unburnt flux. This
would be what happens when you bring the torch too close, and the flux
burns (this isn't happening in the photo)
What is more likely happening is that there is a chemical change in
the one of the boron compounds in the flux. The boron salts in the
flux are a white color. As some of the deoxidation potential of the
compounds are used up, it would result in a small amount being
converted to pure elemental boron which would be released in the flux.
Pure boron is a very dark black color, and even a tiny amount (a
fraction of a percent) would make the flux change to a black color.
This can be differentiated from burning by the glossiness of the flux.
Burnt flux is flat black, not glossy. This is not the same as flux
gradually darkening due to picking up oxides. This would usually
result in a brown (or red/green/blue) color depending on the
particular oxide that is dominant.
The flux will continue to be active, and will work fine. The dark
color could go away on its own, or if you either added more flux, or
stirred the existing flux.
Adding cool flux to a hot joint can result in cracking. A better way
is to either sprinkle on some powder flux, which will stick to the
joint, or to heat some paste flux on a stainless steel spoon, until it
goes clear, then pour a bit on the desired area."
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