Its a track, likely the inside where what little shale was there, has been quickly scrubbed off and now its down to the hard base and tyre rubber starts to create a blue haze.
The track at Arena Essex was like that for its final meeting, saw it close up with my own eyes as we were allowed on it at the end of the meeting, not so much blue but it was like your garage floor, very hard and dusty, I was not impressed.
Proves that whilst technically you don't actually need shale to ride, just that of course if tracks were all like that from kerb to fence they could only run in perfectly dry conditions because as soon as that base gets wet, its becomes like a skid pan.
Shale is good at soaking up rain, remove that and you remove all your redundancy to get meetings on or through to a conclusion.
If they don't care about rain off etc, they might as well just put tarmac down and be done with it, zero cost to keep buying more shale, no preparation required before or during the meeting, no need for tractors or water bowsers, exact same conditions week in, week out.