I read criticism, both literal & implicit, of white people in the workplace who don't, how can I put this, allow for or express sympathy for the pain their black co-workers experience when incidents such as the killing of George Floyd or Deonna Taylor take place. But isn't it presumptous to assume that all black people react the same way to these incidents, that all black people side with BLM?

Sure, if you're close, you know. But if you're close, you're also friends, not just co-workers. Unless a particular workplace is also small & intimate, it's likely that most of one's co-workers know each other on a casual basis at best. Thus, assuming all black peers are outraged & saddened, while white peers are not, is at best dubious & wrong.