Black lives matter … too.
The statement/hashtag doesn’t mean all lives don’t matter. All lives do. But for most lives, that’s long been established. Generations of oppression and marginalization — combined with the recent spate of racial violence that was inadequately addressed in the eyes of many — has the black community asserting that their lives matter as much as anyone else society has deemed worthy of mattering. That’s not only a valid point, it shouldn’t even be a controversial one.
In other words, “black lives matter” isn’t a refutation of any other lives mattering; it’s a way of insisting all lives really do matter.