The usual divide is
1. Colin Kaepernick was a good quarterback who was blacklisted because owners are old white guys who disagreed with his politics vs
2. Colin Kaepernick sucks and was just trying to please his idiot girlfriend and get back in the news.
It's more complicated. My summary statement would be that he was treated prejudicially, and someone should have picked him up. Yet not very prejudicially, and a lot of other guys have been more deserving and not gotten a fair shake. And even had he been picked up, he might not have played much, and certainly would not have lit up the sky.
First, how valuable would he be to a team in a pure football sense? Now, not very much, as he has been away from the game and that matters. But even in the first months of availability, he was a bottom-quintile starting quarterback at best, more likely a high-level backup. Next, he is a running QB, which many teams want nothing to do with, especially after seeing what happened to the more talented Robert Griffin III. Third, he has a definite style and personality of a starting QB, so A)You can't just slip him in as a backup if your starter gets hurt or is so terrible that you have to try the next man up; and B) He has not been noted as a really supportive and easy guy for other QB's, holding the clipboard, offering suggestions in the film room. If he becomes your starter, you have to change your offense on the fly, unless he is backing up one of the very few other running quarterbacks with similar skills. He could be slipped in for Cam Newton. He could step in if Russell Wilson got hurt. Yet even there, those teams would probably be finished for the season if those guys went down. Only in a situation where they just needed a guy to finish a game and maybe give them a chance of winning half of the next few games before the star came back, is Kaepernick useful. Andy Reid doesn't want a guy that can't run what they've designed for Mahomes. Mike Tomlinson wants a guy who can at least temporarily be a Roethlisberger stand-in, even if he might become something else. Bill Belichick had Jimmy Garoppolo and then Brian Hojer - guys who are great in the quarterback room and play a similar style to Brady.
The other side of that argument is that there are teams who played worse quarterbacks at some point in 2017 and 2018. Even if no one said it out loud, there were coaches and GMs who thought, going into a next game or a few games, y'know, Colin Kaepernick would have been a better choice than this guy. That's just true, and I think that's what Kaepernick and his supporters are focusing on. Yet on the other, other hand, at the beginning of a season, or mapping out a grand strategy for the next few years, teams don't think like that. They want a backup who is just a backup because they are committed to their starter, or a backup who is going to take over and be more than a marginal starter. In reality, many of those teams are just bad and they aren't going anywhere even with a hot new quarterback - but they convince themselves otherwise, or have to pretend. They want to roll the dice on Sam Darnold, not try and build a great team that a Colin Kaepernick might, just maybe, if the cards fall right for us, take us to the playoffs.
Summary: Not a lot of teams are going to want him. There are 32 teams, and 24 of those don't need him. Of the other 8, half the teams should want him but they don't. The other half are understandable, given the business issues that follow.
Next, how much do his politics affect owners and GMs? Answer: somewhat, but not a lot. Football coaches hate distractions - witness how ticked everyone has gotten over the years about touchdown celebrations, non-regulation shoes, and players publicly complaining. Do they hate distractions they disagree with more? That ranges from "hell, yeah," to "no, not at all." Belichick benched Julian-friggin'-Edelman for making fun of an opposing coach. Even though the Krafts are big liberals, the only way Kaepernick goes to the Patriots is if they think they can get him for a huge bargain. Like league minimum.
That said, it is likely that a few owners and/or GM's just thought I'm not touching that SOB for any money. Screw him.
Yet I would say those who take it that personally are few. You don't become a billionaire by insisting that the people you work with share your politics. What they are more worried about is how they think the fans will take it. The players are mostly black, and largely at least somewhat sympathetic to Kaepernick - though there is wide variation there. In the NBA nearly all the players are black, and you will not find a coach who has public politics that would offend them, even though a lot of the ticket-buyers and followers are middle-aged white guys. In the NBA, the players hold the power, and their politics are what matter. This is less strong in the NFL, but that element is still in play. There is no owner in the NFL who gives a rat's ass if any of his players votes for communists or terrorists, so long as they don't talk about it, or it never gets in the news.
The league as a whole would prefer that someone sign Kaepernick, to make the problem go away. There is not an NFL-wide plot to keep him out. A lot of fans are black or liberal, and even more of the players are black or liberal. However, the team that signed him would draw the ire of some of its own fans, plus others around the league, and few teams would want to do that. There are loudly obnoxious and bigoted fans who would make it painful for an owner, and some others who were already on the margin and just start gradually drifting away from team support.
I don't think he's treated worse because he's black, but it may be worse because he picked a "black issue." He's poking sensitive areas of the flag and the police. Sitting would have been more disrespectful than kneeling, but I don't know that it would have made much practical difference. I have read some very unsympathetic people shrugging at the end saying "Well, he's standing up for his people," which they weirdly respect. Most critics see him as being divisive of Americans for the sake of identity politics.