All means all, right?

Yes, all means all

As of now: All means "apart from people with dark skin."

That's like saying only black lives matter. That's not what all means

Nope. BLM in context means Black Lives also Matter. ALM in context means Black Lives don't Matter.

There is a long tradition of all not meaning all.

all lives truly do matter, but that cannot be the case until black lives matter

All Lives Matter, but there are those who disagree.

Just because people say something doesn't matter doesn't mean it actually doesn't matter. All Lives Matter doesn't mean 'All Blacks Are Treated Equally', it just acknowledges the fact that every human life should matter equally.