Antarctic and arctic meltwater is "bad" because it's dark, but why is transparent liquid on white stuff so dark?
Water has lowest EM absorption in the blue part of light spectrum and increases rapidly  towards both UV and red parts of spectrum. 



As a result in visible light water is blue. Same goes for the ice as it has very similar absorption spectrum. 

While there is a lot of white in the picture, all of it is a thin snow cover on top of blue ice. Once the snow melts you get a blue water on top of thick layer of transparent blue ice that combined absorb most of the light making ponds looks exceptionally dark. Water would be so much lighter in colour if, for example, instead of transparent ice, you'd have sand on the bottom.