All lives don't matter to a great deal of those folks who claim they do. This pithy T-Shirt (above) ran out of room evidently. All lives clearly don't matter to the All Lives Matter crew. Not at all. Perhaps it is the word "lives" that stumps them. Lives as in life or death. Certainly not as in quality of life. Here they have made themselves very clear - people don't deserve the same quality of life, basic human rights, or privileges. So much so, that the very word "privilege" ignites them. They follow leaders who encouraged them to buy Krugerrands in the 80's at the height of apartheid; they elect politicians who would deny people simple, consistent rights in the workplace; they sue for the right to discriminate freely against the LGBTQ community or to control others reproductive rights; and they pine for the time (MAGA) when they didn't have to work so hard to hate.
You really have to do some soul-searching if you cannot disaggregate these issues. If someone challenges you with a proposition, you should entertain it, debate it, support it, and/or deny it. Once you have done that, move on to your issue. One of the most common logical fallacies is Ignoratio Elechi - whereby one provides a counter statement that does not prove the original statement false. Saying all lives matter does not refute the fact that black matters do - it is a slight of hand that diminishes the uniqueness of an individual or group in favor of a larger collective. Something a card-carrying Bolshevik would be proud of. The real malice of the retort, though, is the visceral pleasure one takes denying the original assertion without actually denying it. Like most cowardly tools, it slithers back into a coarse man's "plausible deniability." I mean it must really suck to be a racist and not be able to shout if from the rooftops (MAGA).
Imagine a mind that prioritizes guns, beer, that Confederate rag, and statues of treasonous villains over seemingly hundreds of core human values ostensibly available to them (e.g. love, charity, compassion, empathy, altruism, humanism (a word under-educated zealots confuse for hedonism), respect, tolerance, joy, justice, hope, service to others, courage, dignity, humility, sacrifice, honesty, loyalty, responsibility, etc.). Now they do dabble in perverted versions of these constructs from time to time, but you seldom see these decent human gestures on their signs or T-Shirts - or hear them from their contorted, malevolent lips. Instead they often embrace contrary explications that make them feel bold, strong, and perhaps, momentarily relevant.
So yeah, if you cannot say "Black lives matter", then you are lying when you say that all lives matter. It's kinda the hateful right's version of participation trophies.
A few words about the rebuttal, "Blue lives matter" - If blue lives matter more than black lives, get rid of that "protect and serve" nonsense.