I think obsessing about penis size is a massive reason men become cuckolds, you worry about it from when your 13 that you have a small dick, then you obsess about how amazing big cocks are and imagining what you could do to women if you had that (like being poor and rich)
But I think to get to the black cock worship, that is part of the journey, you get vexxed by how it seems all black men have massive cocks, then the attraction towards skin tone, muscular features and dominant personality comes into play. Then when you get more into ircuckolding and the idea of bulls being superior and having to give them the love of your life to use and abuse and do things to your partner you cant imagine, and both of you submitting to them, it becomes mythical almost. I have been in love with black cock for about 10 years (maybe more) and about 6 years ago was when I decided to find black to give blow jobs too because it was such a fascination and obsession, I havent done it in ages but the 6 or so black men I did meet a few years ago it was just unreal, I loved doing it so much, and I didn't know anything about cuckolding/humiliation/submission at all, I only used to watch straight interracial porn! So I would say it starts with envy and just evolves to a sacred thing