I just read the MSU fact sheet. When they are talking about 'dark or black hay', they are talking about hay in the middle of the bale.
Grey or black hay is an indication of mold and should be avoided at all costs. Take a bale apart to check the inside; black sections of mold inside the hay indicate bad baling or storing procedures.
Black, gray or white hay (usually caked together) that is moldy is in the center of the bale. It is NOT the discolored hay on the outside that has been exposed to the sun and air. That is perfectly SAFE to feed, but picky horses will leave it if you put out another bale.
Heaves can be insidious and is not easily recognized by most people until it is too late. We have one heavy horse, a 14 year old gelding. He shows NO heave line and is not short of air when he is eating grass or hay that has been watered down. He cannot eat from any round bale including the bright green irrigated hay we have. He just hangs his head and coughs and coughs and coughs and is unridable for several days. He must be kept in and every bite of hay the eats must be watered down with the hose -- a real PITA in the winter. He only shows a 'heave line' if he has been exposed to dust.
And Maura is correct. Once a horse has had any heaves symptoms or if a horse has any allergies, they will probably not be able to eay round bales the rest of their lives. The pollen and mold spores that are normal, even on properly baled hay, are too much for these horses to eat unless they are out in the open. Some, even then, will need to have it watered down like the one gelding we have.
Heaves is real strange in that there are a lot of horses that you can't give heaves to no matter how bad the hay is that they eat a lifetime; and then, there are others that are very sensitive to it and act more like an asthmatic or highly allergic person and cannot even eat round bales without coughing and heaving. I've had horses like this that could not even eat good hay out of a wooden hay manger in the corner of a stall. I had to just throw their hay on the floor or wet in down.