I know this is an old post, but I am new to the site and wanted to add my 2 cents on what I have found on the breed.
In regards to the types: There are multiple "types" of various breeds. Look at the American Bulldog. The heavier, more muscular type are called Johnson type and the more slender "working" bulldog is called to Scott type. Both named after their respective breeders. I think Boerboels are the same in the sense. You get the more agile and slender dogs that did more of the farm work, and the stockier dogs were mainly for guarding the farms from looting natives. For the stockier dogs, a bulk of the guarding ability is looking intimidating. Others were prefered for hunting, and they may have had a different skill set than the other two "types".
In regards to the black Boerboel being a "mutt": I have found that untrue. There is an interview on Youtube with an Kobus Rust, a native South African that grew up with Boerboels and an early member of the largest Boerboel registry SABT. On the interview, he attests that where he grew up, he saw more black and brindle boerboels than any other color. As it was pointed out, the Boerboel is a mix of a variety of European and native african dogs. Many of these dogs have black as their accepted standard. On another website board, there is a discussion on genetic mapping of the boerboels that prove black Boerboels have always existed. It stands to reason that black boerboels were always present in the development of the breed, but were excluded most likely due to personal preference. I have read a short paragraphs about piebald boerboels being prefered for hunting because they were easy to spot in the wilderness. Their color was different from all the native african animals. I can imagine that when hunting its important to NOT kill your own dog.
I find it funny that the breed's "defining characteristic" is its stable temperment, yet most of the arguement about the breed is due to color. Now the big discussion is on whether or not to allow black and tan color dogs. Where are the discussions on hip/elbow displasia?? I think if the orgs paid as much attention to temperment, health, and any aspects other than color, the breed will be in much better shape.
By the way, I own a Boerboel. A 6 month old brown/red male.