Maybe the white people could make signs and chant, "All Lives Matter". On second thought,...maybe not.
White people declaring all lives matter generally mean that lives of people like them matter more. Black lives didn’t matter when black people were placed in chains in the hulls of ships, sailor thousands of miles from their homes, forbidden to speak their own languages, have their own names, marry, read and write, were beaten, raped, murdered with impunity, were bought and sold like cattle, were forbidden to drink from the same fountains, go to the same schools or hospitals, VOTE, marry whom they chose. Black lives didn’t matter when crosses were burned in front of their homes, when their churches were burned, when their children murdered. Their lives didn’t matter when they were directed into substandard housing and substandard loans and substandard schools and substandard jobs. They didn’t matter when Tamir Rice was murdered by police officers or when George Floyd begged for mercy with a cop’s knee on his neck. Or thousands of other times.
Don’t go claiming that all lives matter until the lives of black people actually matter enough that they are treated as though they matter as much as white people.