Blackhand is power-hungry and bloodthirsty, Drunk on the Dark Side before he even takes the fel. But he is still very traditional, and has an honor that Gul'dan hasn't quite been able to get rid of. In this way, he represents the corrupted orcs perfectly.
Related to the above, Gul'dan is the one orc who doesn't think like an orc. He has no honor, no care for tradition, and a lust for demonic power that led to him personally ripping a hole to another world. This is why he needs Blackhand—as seen when Blackhand dies, the Horde would never directly follow someone who acts in such an alien manner. In the original canon, this is why Gul'dan's warlocks ruled secretly as the Shadow Council, with Blackhand officially occupying the post of Warchief.
During one instance of Translation Convention, while Durotan presents his case to King Llane, Garona can be heard in the background translating his orcish dialogue, and she's clearly not speaking English. This makes perfect sense, considering that Azeroth and its history have nothing to do with Earth - of course they wouldn't speak English there. This is something that dates back to WoW's early days, if not before.
In the climax, when Medivh's fel corruption takes over his body, he grows massively in size and grows horns. If you know the lore, that may very well have been the spirit of Sargeras trying to take complete control over him.