It can be demonstrated that black lives have NOT mattered or in improved conditions, have still mattered less than the lives of white people in American society and institutions. The Black Lives Matter slogan functions in a manner similar to slogans from the past which asserted the basic humanity of African-Americans such as "I am a man!" and "Aint I a woman?" 
The basic humanity of persons who are Black must be formally asserted because we live in a nation who WROTE INTO THEIR HALLOWED LAWS and maintained via de facto segregation and other institutions that African-American people are LESS THAN citizens, in fact, less than human. This tradition of racism continues today in many forms, most especially residential and educational segregation. America has not made nearly enough progress in acknowledging and correcting its virulent racism. Thus, America is neither good nor beautiful.
https://en.wikipedia.org/wiki/I_Am_a_Man!
https://en.wikipedia.org/wiki/Ain%27t_I_a_Woman%3F