First, and this is the new bit, but the Ah'Koi Bahnis don't use their word for "matter" the same way as we do. To their more literal point of view, a dead body and a live one have just as much matter in them, and therefore using their word for matter would make the phrase incomprehensible to them. Life is ephemeral, thus it cannot have mass/matter. And they’d probably also be like “what is this black life of which you speak? Life is an abstract concept, and has no color.” So instead of their word for matter, we could use " mah'glohf " because it means "value" in the sense of a treasure. Or really ramp up things and go with " mah'glohrr'if ,"1 which means "priceless treasure." It's safe to say that the AKB would agree that life is a priceless treasure. The AKB are very big on the right to bodily autonomy; any violation of bodily autonomy sends AKB into a frenzied anger, and murder is the ultimate violation of bodily autonomy.
Second, Ah'Koi Bahnis come in a vast array of colors that humans don't (practically any color or color combo you can think of), so if you tried calling a black person their word for the color black ("mor'shek'iss") - or our word "black", if they knew English but lacked the cultural context - they would assume you meant someone whose skin was coal black or darker, and would not include people with brown skin in that. But you couldn't really call human black people brown, either, because to the AKB, most humans are some shade or other of either brown or red, so it would be too broad a word. (They even consider the average Caucasian skin tone to be various shades of brown or red.) You could make a translation to "Da-Nykahr seh da-bahn guu'tik2 jokiidj Ah'frih'kah3 ulg mah'glohf," which means "Lives of brown persons from Africa have value," but that's kind of unwieldy.
Then I realized, in the future in which the stories take place, the AKB will have become familiar with the human concept of race (even if they think it's a bit silly to base race off something like skin color), and probably have a word for human black people. I don't know how often they'd use it, but if the distinctions between races were important enough to enough humans, probably often enough for it to be well known. For that, I came up with "Ah'frih'guu'tik," meaning "Brown people of Africa. "Da-Nykahr seh Ah'frii'guu'tik ulg mah'glohf" is shorter and sounds better, doesn't it?
However, I think the spirit of the "Black lives matter" phrase would lend it best to the following translation: "ny'Ah'frih'guu'tik bain mah'glohrr'if." Translation back into English: "[Lives of black people] [are] priceless treasure." And also interesting is that in TPNN, the expression can be either singular or plural. You could specify plural by adding a "da-" prefix to "ny'Ah'frih'guu'tik," but it's not necessary.
And if someone came back with "ALL lives are priceless treasure," they'd likely respond, "I'm glad you agree with me." Can you imagine how flustered that would make the racists? Though the AKB might also have to respond with "You say all lives are priceless treasure, and that is good if you truly do believe it. But you need also to realize that a great many of your people do not treasure the lives of black people, a fact that is self-evident from the horrendous ways in which they are treated every day. Ways that differ greatly from how 'white' people are treated."
LOL, while writing that, I figured out what the AKB would call white people: Yew'rahp'dwah'yoot. Which means "Pink people of Europe." Though the word for "pink" in TPNN literally translates to "small-red," so you could also translate that as "Small-red people of Europe."4 Which, while funny, is a confusing translation.
Sure, AKB speakers could easily say "Yew-rupp" and could likely say "Yer-up" without much difficulty, but a lot of AKB language is about what sounds good, and I personally think Yew'rahp sounds better then Yew-rup or Yer-up. Especially since those two don't really mesh with the pronunciation of "dwah'yoot" when I say it. Trying to say "Yer'up'dwah'yoot" feels/sounds like switching from one language to another mid-word.
1 = mah'glohrr'if is not an easy word to say, though. The double R ("rr") is a roll of the tongue like in Spanish, or like that old Rrruffles have rrrripples thing.
2 = guu'tik is pronounced goo'tik.
3 = TPNN has an AA as in Africa phoneme in it, but as I can't think of any words with that phoneme, I've long since come to the conclusion that English words with that sound would be mis-pronounced as AH in a TPNN accent. Hence "Ah'frih'kah." (Reminder: apostrophes in Latin spellings of these TPNN words just separate syllables.) I will note, too, that some TPNN sounds tend to shift pronunciation slightly, so you're just as likely to hear a TPNN speaker say "Ah'frii'kah," pronounced "Ah'free'kah." It's like the R/L thing in Japanese.
4 = And extremely pale people of European descent might be called "Yew'rahp'yoo'tii'koo," meaning "White-red people of Europe," if not for the fact that Yew'rahp'dwah'yoot already includes them. Useful distinction, though; I'll keep it. And I'm sure the AKB would want a word for gingers, too, so I give you "Nng'gyuh'yew'rahp" for white gingers and "Nng'gyuh'guu'tik" for all other gingers. Nng'gyuh means "spotted" as in "has spots." So the names respectively mean "Spotted people of Europe" and "Spotted brown people." Or you could specify black people with "Nng'gyuhf'rih'kah." I have no better idea how to spell out the pronunciation of Nng'gyuh than to say it would be spelled, in TPNN's written syllabary, as the character for the letter N with a mark on it that means "draw out this sound for a second," then "gyuh." The reason for the discrepancy in the structure of the words for gingers is because they sound better that way, and since it doesn't violate any rules of the language, why not switch it around?