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.

This is like saying being "anti-murder" is countercultural because murders still happen.

Just because something occurs does not mean there is no mainstream opposition.

I would go further and say that the most basic elements of the BLM were never countercultural.

opposition to excessive police force has been mainstream since the 70s-80s.

The only part of BLM that is actually controversial and countercultural is the idea of abolishing the police. As a result, the majority of BLM supporters reject this aspect.

Note that by that measure, hippies were not counterculture.