What would be the cause of blue tint, hue, specks  etc to be visible on the surface of solder joints. Is it just alloy composition?

I have seen this in the past typically in high temp reflow /high lead alloy, copper lead frame devices around the die attach fillet.

Now it is being seen on a 63/37 alloy solder smt reflow.

It was not seen in the past, the cleaning chemistry is buffered to protect the metal etc. and has been in use for many years without seeing this phenomena!

Maybe if a crappy traditional high ph saponifier was used and the solder joint was dull and gray it would not be as noticeable?

Perhaps the copper is leaching to the surface from the tinned component lead or HASL surface?
My suspicion is that trace amounts of copper is present on the surface of the solder joint after reflow. That when exposed to reflow temperatures it picks up this bluish color.

Charlie