Define "BLM", "protests", and "success" because any combination of different variables produces a different result. Additionally, even then, there is a lot of nuance to being successful when it comes to political movements.
The protests undoubtedly brought more attention to policing and racial issues in general. They obviously didn't solve either problem. Some states passed progressive policing laws, some regressed out of spite or in reacting to the other states.
Then you also have the category of "well, it might have made an impact on this but we'll never know". For instance, does Biden win in 2020 without the Black Lives Matter protests? No idea, and nobody truly does or even can. That would be an enormous impact on many things, some of which may not even have been goals of the protests.
I think a better question to ask is whether the groups and ideologies involved in the BLM protests (which were MASSIVE) were ever allowed to have power?
If BLM failed to enact significant policy change than I don’t think it is because BLM wasn’t focused enough, had unrealistic goals or was handled badly, I think it is because in terms of law enforcement policy it really doesn’t matter what voters do or don’t want. Any kind of noise made by voters and the public about police violence and the inherent problems with police (and their vital role in maintaining economic injustice and inequality through state violence) will be aggressively pushed back in the opposite direction by the political forces of law enforcement, and because the average person has no power and their vote is useless this will result in a broad push in policy in the opposite direction of BLM’s goals.
However, the function of BLM must be seen for what it was then, to lay bare the true nature of the power relationship between voters and cops and in the minds of countless, countless people living in the US it delegitimized the authority of law enforcement to commit violence wherever and howsoever it chooses. It sent a massive crack through the entire structure of policing, jails and systematic divestment from minorities and the poor. Just because BLM didn’t create significant policy changes doesn’t mean that the battle hasn’t already been lost for the legitimacy of law enforcement in the long term in the US, and I call that a victory.