It's not a matter of simplfied proportions. The only justified argument on that side as of now should be: "Police bias vs. blacks in stops and use of non-lethal force does not carry over to the use of lethal force."
If BLM focused only on non-lethal force and harassment, there's no data that can be used against them, that I know of. But that doesn't generate as much outrage.
Stop and Frisk and non-lethal police violence protesting isn't sexy enough for news headlines and click bait attention whoring, obviously. Murders caught on video are juicy. Didn't you all watch Nightcrawler?