What you are seeing is a change in surface texture. This occurs when the solder transitions from liquid to solid. In the process, the solder forms into lots and lots of tiny grains, and the surface becomes rougher than it was as a liquid. This surface roughness reflects light in all directions, and the result is a surface which is not dark (since ambient light is also diffused) but which looks dull. These grains are not crystals as such, but rather occur due to impurities in the solder, which form dislocations which prevent the solder from forming into a single crystal in the process of solidifyng. In general, this occurs in any impure substance which is cooled below its melting point quickly. If you were to take extremely pure solder, melt it into a vacuum to prevent surface oxidation, and then cool it very slowly, you could produce a material with a very smooth surface and high reflectivity. It wouldn't be a perfect surface, since solder doesn't have a defined crystal structure, but the grain size would be much larger.