I watched the construction of the track. Underneath are several inches of asphalt, which is oil based. When the weather gets warm and the oil heats up, the churning of the material makes it rise to the surface and blend with the poly.
I believe that is why it has turned so black.
That would count as crapola that's been absorbed into the poly fibers. Oil, dirt, dust, horse poo, leaking transmission fluid from tractors, whatever. Having all those fibers totally permeated by oil to turn them black has got to change the performance characteristics, one would think.
It would be interesting to see the results of some actual scientific compression tests or whatever of the track surface material now vs. that original lighter fluffy stuff. Heck, the fibers don't even LOOK the same any more when you see them up close, down by the rail. Much smaller and more shredded/beat up.
Of the official "Polytrack" brand synthetic surface tracks, I think Woodbine, Turfway, and Del Mar have all changed theirs out at least once since original installation. Don't know about Keeneland. There can't be an indefinite life span to them.