Should he have been fired?

If caught they were punished most never tried that doesn't mean they were dumb all that means is that they fear the punishment of trying that's more conditioning then anything else. From a genetic stand point that doesn't make them have any less aptitude for intelligence. You could take any one of those slaves and teach them how to read and write.

You're mixing arguments here.

If you actively try to weed intelligence out of a population with absolutely no concern for whether or not you over do it, you're going to succeed to a significant degree in both reducing the intelligent population, and making the ones that survive more capable of hiding it.

That fact aside, the fact that they can be taught to read and write does not have any reflection on their intelligence. The mentally retarded can be taught to read and write. They're not considered to have higher intelligence. It's only in cases of extreme deficiency that you lose that functionality, and slaves weren't wanted quite that stupid, because they needed to be capable of retaining and following instructions. They were targeted for the low end of average. There were definitely incidents of significantly retarded slaves as well, which does sort of blow your 'any' claim out. Those slaves were more limited in what they could be used for and less likely to breed though and naturally weeded themselves out more often than not.

Even if one in one hundred of them were potential geniuses they would've had little to no way to actually foster that spark into something meaningful, or to pass it on to offspring. At the genetic level intelligence isn't a magic number that defines who you are, at best it's an indication of your natural predisposition towards how quickly you assimilate new information, how you relate it to existing information and how readily you can recall and correlate it. A person with an IQ of 70 can still learn everything that a person with an IQ of 150 can, the higher IQ is just capable of learning it more quickly and using it more readily. A person with a lower IQ and determination can easily equal or exceed a person with a high IQ and little motivation/interest. It's not a cap on how much you can learn or know. It's not even a fixed number, it's one that can expand or atrophy as the brain itself evolves and adapts to how it's utilized. In the case of slaves and their offspring, a lower IQ was the norm because they were not given situations or tasks that would've stimulated them. Lacking parents with a higher IQ children are less likely to be higher, and lacking people in their lives to stimulate and encourage that sort of growth, something slaves certainly didn't have, there was no catalyst to spontaneously develop them. Throw in post emancipation education, which was shoddy at best while segregated and is pretty much still crap now, and as a culture the average African-American has not had the opportunity to be naturally blessed with the higher IQs of the privileged upper classes and have to strive significantly to keep up or exceed. Those who've been able to, through sheer determination or luck, get ahead and develop themselves are often ostracized by their peers, who either haven't had the same luck, the same determination, or in some particularly sad cases have bought into the cultural lie that the world owes them something and they don't need to try.

I think you're taking it personally because of how it makes you feel to be told your ancestors were "dumber" than average, but that's silly. Almost all of our ancestors were dumber than the average person today, because none of our cultures was as advanced as we are now, or had the opportunities we do today for education. Yours was just more recently repressed than most. That's not a reflection on you, or them. How you evolve going forward however is. Now that you have the option to grow and expand as a culture if you fail to utilize it, or create excuses for why you can't, that's all on you, which again is something Cosby and Oprah have both preached, as have many others.

I fully believe that anyone, black, white, red, yellow, green or purple, anyone can accomplish the basics - graduate high school, get a degree or trade certification, have a decent job, raise a family and contribute to society. As long as they're willing to put forth the effort.