Manumuskin,
Keen observations! Natural blue holes have pretty much faded from memory (e.g., Blue Bent Pond, Dog Heaven, Danger Hole, Three Pond Holes), and many of the features we call blue holes today are simply relicts of sand mining. I think their water-coloring mechanisms are similar. If silt or clay suspension were responsible, then one would expect the blue tint should fade with time. If a biological agent were responsible, then the blue tint should increase with time. Your observations seem to support the latter scenario.
Whether or not their presence lowers the shallow groundwater table remains debatable. Neither Rhodehamel (1970, 1973; hydrologist who calculated 17 trillion gallons of water within the Cohansey Aquifer) nor Rasmussen (1958; hydrologist who studied Delaware’s spungs) were able to calculate the water budget (+ or -) of small basins. Rhodehamel (1970: 10) was able to estimate that approximately two percent of the Pine Barrens is covered by closed basins. I wonder how much the coverage area has changed in four decades…
Spung-Man
Rasmussen WC. 1958. Geology and Hydrology of the “Bays” and Basins of Delaware. PhD dissertation, Bryn Mawr College, Bryn Mawr, PA, 206 pp.
Rhodehamel EC. 1970. A Hydrologic Analysis of the New Jersey Pine Barrens Region. Water Resources Circular. No. 22. State of New Jersey, Division of Water Policy and Supply. 35 pp.
Rhodehamel EC. 1973. Geology and Water Resources of the Wharton Tract and the Mullica River Basin in Southern New Jersey. Special Report No. 36, Division of Water Resources. Trenton, NJ: State of New Jersey Department of Environmental Protection. 58 pp