Michael Henchard's face is often described as a combination of red and black. Weird as that sounds, we're probably not supposed to imagine that he looks like Darth Maul from Star Wars. Although Henchard is racially white, he's very tan, and his work in the outdoors has turned his skin ruddy and red. Folks used to describe being very suntanned as being almost "blackened" by the sun.
Henchard also can't hide his emotions, and his blood is always rushing to his face when he's angry or upset ("Henchard looked at him with a face stern and red" [15.34]). This also helps account for the "redness" of his face.
What else could the red and black coloring suggest? Let's check out another example:
Elizabeth-Jane now entered, and stood before the master of the premises. His dark pupils – which always seemed to have a red spark of light in them, though this could hardly be a physical fact – turned indifferently round under his dark brows until they rested on her figure. (10.10)
This description of Henchard makes him sound pretty scary. Dark eyes with a "red spark" in them sounds almost demonic. Is the narrator suggesting that Henchard should be associated with the devil? Not in any literal way, but perhaps this description is meant to imply that Henchard has very little control over his own metaphorical inner demons – his bad temper and his pride.