At this point, it should be obvious that there are non-football forces working against Colin Kaepernick finding a job in the NFL. The Ravens explicitly said they wanted to check with sponsors and fans before considering signing him and ultimately decided to ride with Ryan Mallett.
But there are also football reasons why Kaepernick is not getting signed. Some teams either do not like Kaepernick as a player or do not believe he fits into what their team wants to do from a scheme perspective. In a piece directly designed to counter the "blackball" theory on Kaepernick, Albert Breer of TheMMQB.com spoke to multiple NFL executives about their decision to decline pursuit of Kaepernick for their team.
One executive explained that he doesn't think Kaepernick "can play." From Breer's article:
"I don't like the guy as a player. I don't think he can play. I didn't think he could play at Reno, I don't think he can play now. … You don't think if he was a good player, 20 teams would be lining up? … He's inaccurate, inconsistent reading defenses. He needs everything to be perfect around him, and he needs to run a certain offense. When he was rolling, they had an unbelievable defense and a great running game with an amazing offensive line. Everything was perfect. And you consider that, why isn't there a debate about RG3? He just wasn't a consideration."
Another executive said that Kaepernick's name didn't come up in his organization because "Kaepernick almost has to be in a place where they'll build a system for him, and teams don't do that for backups."
There is some truth for that: Seattle felt like a good fit for Kaepernick because he could theoretically run a similar offense to Russell Wilson, or at least play a similar style of football. The easy counterpoint would be that Wilson, who used to be Kaepernick's chief rival in a heated 49ers-Seahawks rivalry, might not want Kaepernick on the roster behind him.
A third executive echoed those sentiments, claiming it was a schematic situation that caused the team to pass on taking a look at Kaepernick.
"To me, the protests, all that, it wasn't even a factor for us. It was the ability to fit within our offense," the executive told Breer.
The bottom line here is that some teams probably do not want to sign Kaepernick or probably are not interested in signing him because of his fit on their football team. But, as the Ravens essentially proved when stating they would ask fans and sponsors about signing Kaepernick, there are also football teams not signing him for off-field reasons. Baltimore is actively choosing to let Mallett be a factor in their 2017 season instead of upgrading to Kaepernick. That is a non-football situation.
It can be both things and people will be mad about Breer citing anonymous executives, but there are certainly people in the NFL who do not want Kaepernick on their team for football reasons.