Hopefully, this will not to add to the confusion. I do restoration work in the Denver area and have seen this numerous times with this stone. I believe it is the resin darkening. I have been successful once with using an acetone pultice, numerous times. Soak a white rag or paper towels with acetone. Cover with plastic wrap and tape it down. Repeat often. Good luck.
if the resin darkened, how would a poultice help? resin is not porous. i have seen resin turn colors, but have not seen any methods reverse it. the stone itself can absorb and darken then surface, which we have dealt with a few times.
Diann, great to hear that they got it done for you.