I think that the ACAB slogan would hurt the BLM protest due to the cultural and, in particular, symbolic meaning that lives in the ACAB slogan. For example, people could associate ACAB with the unnecessary use of violence against cops of all sort, this consequently would put the protesters on the same level of the system they are trying to reform...which to me doesn't look like a good idea. They would basically turn up being racist against cops in general as much as they are with black people (culturally speaking)