Wally: no real mystery here, It is a spring, that feeds the pond, separate from the stream. The clear ground water contrasts with the more nutrient/sediment laden stream flow, If one had visited during the winter, one would have seen this same area unfrozen from the rest of the pond, again a "clear" indication of the spring. As to this area after the dam is removed, as I understand, this this area is deeper than the "pond" of the dam and will remain, as a small pond and new tributary to Hamant Brook, although only time will tell. The clearer water reflects the blue sky better, thus the color. The ponds have already started to show signs of an algae bloom, thus reflecting less blue sky. I also believe that some time in the past, the Hamant Brook stream channel was relocated from approximately this blue lagoon area to its current route slightly to the west. My training in riverine ecology leads me to believe moving surface flow is easy, moving sub surface flow is not so easy.
I'm no expert, but this my best guess.