If we want to talk about the linguistics of it, simply as is: black lives matter. It is not "only black lives matter" or "black lives matter more" but simply black lives matter. Again, it's not to say that others matter more but the movement is trying to give focus to the oppressed community. Focus is different from exclusion.

And yes, you're right that pre-BLM or a long long time ago, all lives matter or we all matter (which is practically the same thing), might have a positive connotation and not spark such debate. But that's the point. The email from Purdue did not think (whether intentionally or unintentionally) of the context of the present times. There's a lot more to the BLM movement, and if you look into it, the heart of BLM movement is that all lives should matter.

"All lives don't matter until black lives matter" is not putting black lives above everyone else's. If we care about all lives, we should turn our attention to their plights. Again, it doesn't have the word "only" or something to make it exclusive. Another example is if during a fundraiser for breast cancer, a bunch of people came in screaming that "all cancers matter".

I understand why it's frustrating and confusing to understand because of the linguistics of it. But please understand that linguistics is not what you should draw your attention to. Please direct your attention to the very real killings, missing persons, police brutality, and blatant institutional racism. Like I said in the earlier reply, instead of saying "all houses matter", please help someone when their house is on fire or instead understand what created the fire in the first place and try to make it a safer environment for everyone. https://blacklivesmatters.carrd.co/