I grew up in an ALL white congregation, and not in a place you would expect either. It was not somewhere in middle America, it was in a BOROUGH OF NEW YORK CITY.
The area I grew up in, and I am from, was a predominately Italian American population. Hence, so was the congregation I was in from infancy. In fact, there was even another congregation that met at our hall, that was only Italian speaking.
That being said, it was in a VERY affluent area, literally multi million dollar homes everywhere, including on my block growing up, AND the same block as the Kingdom Hall. This created the same type of "pitfalls" that you ge, like the OP mentioned.
Yet, in the same borough of NYC, ALL the other congregations were mostly black and Spanish. In those congregation there were actual families living in utter poverty. Some families did not have enough food to feed themselves, or enough money to cloth themselves.
In those congregation, however, I would not say they were necessarily more zealous, or free of "sin and problems". In fact, in those congregations there were quite different issues. Many of the members lived in NYC housing projects. This meant that the Elders there were dealing with members whos kids were joining gangs, selling and doing drugs, and involved in crimes that sent them to jail. Also, the morality or lack of, seemed to be just as prevalent in those congs, too. Kind of equal in all.
As far as out and out full blown racism, I really did not see that from congregation to congregation. When black brothers, and their families visited, they were treated like everyone else, including being invited over for dinner, lunch, and that kinda stuff. In fact everyone was would get a dose of the FAKE, PLASTIC, CONDITIONAL "LOVE", that we know most JW's are good for.
Now, there was a sense of superiority, in my cong compared to others, but I believe this came more from an economic standpoint, not so much a racial one, (not saying that's better). Also, my cong was mostly Italian American, and with that comes a certain "clan" like mentality, that is just found in certain cultures. This, of course, was in the cong, and the "world".
I believe this kind of stuff exists on all different levels, and in most congs. Not necessarily racist, but a certain "us and them" type of attitude, but mostly from a financial standpoint. In my old cong I knew of white brothers marrying black sisters, and vice versa, but I also knew of white families that were assigned to "black" congregations who transferred to my old "white" congregation, even though they didn't live in its jurisdiction. I don't know what their reasons were, but it could very well have been racial, or wanting to be more "comfortable", as some would say.
Also, many Bethelites would go to my old congregation, and they were mostly white, and "higher ups" with a lot of pull at Brooklyn Bethel. Did their "status" allow them to pick my old congregation? Why wouldn't they go to the others, where possibly they were more needed?