Anecdotally, if you look at a crowd of protesters the media tells us are #BLM protesters, it sure seems like a lot of them, even a majority, are white.
We also have a first hand account from a Black cop about how a white #BLM protester did everything in their power to stop a black #BLM protester from speaking with that cop. (
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Some quick research on the interwebs shows a Pew Research Center survey that would also seem to support the notion that Black "protesters" make up only 17% of those who attended a protest focused on race or racial equality.