Greetings! It appears that your requested Doctor is offline and I didn't want you to have to wait any longer for some assistance.
Sorry to hear about Lex's (and you other horse?) condition. The most common reason for black manure is caused by blood from the upper G.I. tract, such as the presence of stomach ulcers
. Stomach ulcers are very common in horses, even when they aren't overtly showing clinical signs. Your Vet should have a stall-side test to determine whether or not there is indeed blood present, at which point I would start them on an ulcer treatment. There are various (and often very expensive) current ulcer treatments, which are only available through your Vet. One treatment that I have had a good amount of success with is a mixture of Aloe Vera Gel and slippery elm powder. I normally recommend 60cc of the gel, with one teaspoon of the powder mixed in. Normally given orally once daily for 7-10 days, with very often significant improvement. This is in my experience only, and as I can't test your horse for the presence of blood, your Vet would need to do the test initially, then the treatment type would be your choice.
There are also at times ulcers present in the upper small intestine, and a definitive diagnosis of either stomach or intestinal ulcers requires the use of an endoscope, to look directly into the stomach and intestine. Most Vets do have access to scopes these days, so it might be worth pursuing should blood be present in the manure.
I also, like you, doubt that it was the change of hay that caused the black manure, but there is also a possibility that they are eating something that you are unaware of that is causing the color. The only way to know is to run the test for fecal blood.
So I trust that I've been of some help, and please let me know if you have any further questions.
Many thanks, ***** ***** of luck!