If you recall, that question was asked of the Democratic candidates during their debate. Every one answered it by saying that black lives matter, except for Jim Webb, who replied, "As a president of the United States, every life in this country matters," an answer for which he was roundly criticized by other Democrats.
To make the logic of the question clear, suppose it were asked: "Do some people's lives matter or do all people's lives matter?" If the response were, "Some people's lives matter," the implication would be obvious -- that some people's lives matter, but not all.
So the implication in saying that "black lives matters" rather than that "all lives matter" is that blacks are to be considered separate from the rest of humanity -- that they are a special class of people to be treated preferentially. The racist implications of that are stunning, but apparently not to Democrats, who have long endorsed preferential treatment for African Americans.
A century from now, historians will look back at the political dialogue of 21st Century America and wonder how a people so obsessed with egalitarianism could become so racially polarized.