The Black Soldermask is used when light reflection is not wanted
Solder Mask is one of the most important materials in PCB production. Solder mask or solder resist is like varnish or lacquer that is applied to the outside or top and bottom layers of a printed circuit board to form a permanent protective coating for copper traces. The solder mask prevents solder from bridging between conductors and creating a short in the intended circuit. It also provides some protection from the environment.
The solder mask is most often applied with a green tint but is available in a wide variety of colors and finishes. In ALLPCB.com, these colors are available: green, black, and white, yellow, red, etc. Black Soldermask is used for some displays systems where light is not to be transparent. That’s to say, black soldermask is used when we don't want light reflections etc.
Black Soldermask
The contrast between traces, planes and empty space is virtually non-existent. Inspection of the board not only requires powerful magnification but you also have to angle a light. The silkscreen contrasts well.
The black solder resist absorbs heat during reflow. You have to either scale down your profile or make sure that your temperature sensor is actually placed on the board itself. The silkscreen is also prone to turning light brown during reflow, presumably because the board under it takes on so much heat.
