I'll believe BLM is mainstream culture when people of color aren't being disproportionately killed by police.

Until then, if police forces are upholding a mainstream concept of law and order resulting in said disproportionality, BLM is countercultural.

Otherwise, if acknowledging the aims of basically what's a human rights movement via flags and spots on AP tests constitutes inclusion into mainstream culture, then the mainstream culture is absurd and perverse.