Are blacks oppressed in the 21st century?
They are, It's pretty clear, the subject of systemic racism by police and the justice system. This is especially true in inner cities, and anybody who doubts it can study the Ferguson report for evidence. That being said, there are a variety of factors that explain why this is so. By refusing to stand for the flag, Kaepernick is essentially blaming the USA as a whole country for this racism, and also implying that it is deliberate. Those are unwarranted assumptions, IMO.
Are blacks oppressed in the 21st century in other ways: economic, social advancement, culturally? These are much more difficult to judge, but despite the assertions of Donald Trump (and apparently Colin Kaepernick) I would say no.
But even if you're one of those who believe they are oppressed both by the police AND the economic system, any rational person would have to acknowledge that in all areas conditions are far better now than they were 30 years ago, and those far better than 60 years ago, and those far better than 90 years ago. One of the most remarkable aspects of this nation, which I proudly consider the greatest in the history of civilization, is that we keep getting better about this stuff. We move forward and improve our attitudes and our results, making us today a better country than we've ever been, even though we have a long way to go yet. This continual improvement, yearning to be better, is one of the major reasons I will always stand for the flag.