Well, if the screenshot of Blackheart with blackish skin, spikes, fangs and red eyes is the true form in the movie i'm okay with it. Really, he can mutate form in comics, like its father, i saw several forms of him over the years so i'm not even disturbed by this. From the description of the scenes i think it stand faithful to the overall look of father and son over the movie, a giant cg figure would have been way to fake, like malebolgia at the end of spawn(whichisa stupidGRripoffbythewayyouknowmayhebedamned). They still have the eyes, spikes and colour spot on, that's is enough for me, and i am a long term GR fan... maybe some suggestion of a tail when he emerges form shadows, ro some hint of it in some scenes, that is my only suggestion i think could have worked.