....."It just has a slightly blue tint you can see in the sun. It is otherwise dry and normal looking.".....
>>>>Maybe a misting of fly spray wafted over the pile? Something from a neighbor's yard on a windy day? Pollen?
Not trying to be curt, but IMO you need to look at the big picture: if your mare is otherwise doing relatively well and "blue poop" is really the most pressing thing you are concerned about at present, then you are darn lucky. Every single horse on this forum is ill in one stage or another. Some are downright critical and may not survive until tomorrow. I'm not at all belittling your mare's manure situation, so do make daily note of the blue poop (hopefully you keep a journal of sorts for observations?) and see if you can find correlations between the poop color and any other change (such as supps, meds, new bag of foodstuffs, dewormings, vaccinations, changes in weather, new hay, etc). Perhaps you can ask your vet if its worth sending a manure sample for testing, if you are still concerned? Your local vet may be more helpful deciding what lab would be most useful for your area of the country.
Lastly, and most importantly, please do take the time to hug & appreciate your mare every single day and revel in the time you have left while she's relatively healthy.
Best of luck,
Kerry in NY