Abaddon would be caucasian like Horus Lupercal, the Warmaster. Abaddon, prior to being despoiled by the warp, was said to bear one of the closest resemblances to his Primarch. Most space marines usually bear at least a little resemblance to their Primarch from the Gene Seed changing their features. Here's a quote from 'Horus Rising' by Dan Abnett, I looked this up in my paper copy!
"Ezekyle Abaddon, first captain of the Legion, was a towering brute. Somewhere between Loken's (SM1) height and Torgaddon's (SM2), he seemed greater than both due to the cresting top-knot adorning his otherwise shaved scalp. When his helmet was off, Abaddon bound his mane of black hair up in a silver sleeve that made it stand proud like a palm tree or a fetish switch on his crown.... He, like Torgaddon and Aximand (SM3) both, shared the wide-spaced eyes [and firm straight nose] so reminiscent of the Warmaster..."
In all the official imagery, such as on the cover of this book, the SMs of the Luna Wolves (who went on to become the Sons of Horus and then finally the Black Legion) have black hair and a warm caucasian skin tone.
This image is from an audio-book version but it's the same cover as on my book.