I think one it turns black the only way to reverse the situation is disassemble and clean everything. Just change the fluid once a year and don't worry about the color.
All my ducs (not that many, but anyway ....) have been like that. In the begining, I worried about it and changed the fluid, but with age-induced laziness setting in, fluid changes started happening only when made necessary by other causes. So far, I´ve never noticed a difference this way or that.
Infrequently, if not renewed as per above, I do pump thru some fluid and top up the reservoir, but that´s it.
May not be the correct way, but it has worked so far