Interesting way to put it- you are totally right.Just for the record, I don't think most of black culture is SJ. Like I said, circumstances forced a diverse group of people to homogenize and lose their cultural distinctions and but also reform another. For the sake of survival, black people have had to stick together because of the amount of common problems and obstacles. There's had to be a wholeness because if you were fractured as a group you wouldn't have as much power. Of course black people are hugely influenced by larger society but then again influence is multi-directional.
I believe MBTI is a culturally insensitive instrument, so it's not surprising to me if most of mainstream culture is typed SJ then those same markers would be used in black culture to label SJ. If for example, being overly-religious is a marker of being SJ (valid or invalid, true or not) then most people would use that indicator to type someone as SJ. I know quite a few black intuitives that are very religious and I'm the heathen according to them.
I think MBTI does a lousy job of translating what functions are in different contexts. I don't think a black SFJ presents the same as a white SFJ. I just don't. I can tell when I'm in a group of black SFJs and when I'm in a group of white SFJs but the similarities between those groups are just majorly different. Frankly I do think black SFJs appear more SP than white SFJs. Same thing goes with sensing and intuition within black culture and white culture. White people tend to use level of weirdness, bizareness, and inattentiveness to surroundings as a measure, but I don't think that's an accurate measure in black culture.