I hear what you're saying, but I think you misunderstand what the protesters are saying.
“Black lives matter” is not just asserting that black lives are among the lives that matter. Like, it IS asserting this and that observation IS somewhat banal and that IS kind of the point.
The banality of the statement is meant to invite you to think, “wait, why do we even have to say this?” which exposes a deeper meaning.
That deeper meaning is more like “We occasionally get hunted and killed out in the streets like rabid dogs by the very institutions that are supposed to protect people from violence—and this can only be understood as the culmination of America’s vast history of trying to ignore and bury and forget about its racist history rather than talk about it and address it, to the point where that «we don’t wanna talk about it» inclination has metastasized into an active implication that our lives are valueless to the culture at large. And we cannot breathe this stifling choking smog of «oh well another black man was shot but let’s not put it in the news, they don’t matter enough for that» any more, as happens when death transitions from stories into statistics. Fuck that. We DO matter.”
In turn the deeper meaning of “Blue lives matter” is “we respect our police enough to give them unconditional arbitration of who lives and who dies in cases where they feel their lives are at stake and sorry black people but we don't see a way for your needs to not die be satisfied without good honest police officers losing their lives because some of y’all shoot them—like, not the majority, I am sure most of you are good, but like when I think of inner city gang members shooting the police I think of black men stereotypically. Fix your gang problems first and then you won’t get shot by police.” You can see why that seems to have a kernel of truth but really came across as tone-deaf (see how it takes a statistical view rather than a story view?) and not understanding that most of these actual stories are about getting shot in the back while walking away or being strangled slowly while being totally immobilized and protesting that you are being strangled and could they please not do that, others involve very young innocent children being killed as collateral damage or worse.
Meanwhile “all lives matter” is a similarly banal statement but it serves the function, when used as a response to “black lives matter,” of saying that “no, you know, I really like our racist history being buried, I liked it when we didn't talk about race. We should focus on the global human condition and forget the specific gruesome stories of what’s happening to folks of your race right now as just the smaller problem of what’s happening to you right now. But I find myself very worried that if I am sharing these black stories someone is going to get mad at me for not sharing the Mexican stories of police injustice and then the poor white stories of police injustice and, well, can’t we just go back to a time where we didn’t talk about the race problem that we were having?”
The basic response to this I suppose is “we have been trying the ‘all lives matter’ approach for hundreds of years and we don’t seem to be improving, meanwhile cultures with similar race dynamics, like South Africa, are actually processing their difficulties over much shorter time scales, possibly because they are willing to talk about it and be frank. There is no reason to believe at this point that the mentality of ‘all lives matter’ leads to a better outcome for the black lives that are hemorrhaged today in such quantities that it becomes numb statistics rather than individual stories if we don’t make a point of getting outraged over every single damn death.”