Hello nnms. Not sure of the protest part. Speculating is they are actually mourning a loss and the grieving process is black used to protest that loss. Anger is the second phase of today's accepted psychology of grieving following denial. Next, comes bargaining, so possibly still a parallel with the union of the protesters negotiating. Depression follows and then acceptance.
With the color black a short peek offers it is not universal for mourning. In Asian countries it is white. White has dotted Western culture for mourning too, even though black goes back to the Roman days. It seems it may have more to do with the cost of dye and being all the same in color. I saw one article briefly discussing the absence of moonlight as symbolic too. More mythical in thought with that maybe.
Thanks for the question and do have a great day,