If Alive Today, Would Mark Twain support "Black Lives Matter"?
I believe that he doubtless would.
By exposing the way some white folks thought at the time (mid-1800s) and place (Mississippi River valley), Mark Twain made the point in "Adventures of Huckleberry Finn" that Black Lives Matter.
You might even say that is the whole theme of the book. For one example of that, note this passage from Chapter 33 where Twain, in a tongue-in-cheek way, underscores the illogical thinking of some white people of the time and place:
“Now I can have a good look at you; and, laws-a-me, I’ve been hungry for it a many and a many a time, all these long years, and it’s come at last! We been expecting you a couple of days and more.  What kep’ you?—boat get aground?”
“Yes’m—she—”
“Don’t say yes’m—say Aunt Sally.  Where’d she get aground?”
I didn’t rightly know what to say, because I didn’t know whether the boat would be coming up the river or down.  But I go a good deal on instinct; and my instinct said she would be coming up—from down towards Orleans. That didn’t help me much, though; for I didn’t know the names of bars down that way.  I see I’d got to invent a bar, or forget the name of the one we got aground on—or—Now I struck an idea, and fetched it out:
“It warn’t the grounding—that didn’t keep us back but a little.  We blowed out a cylinder-head.”
“Good gracious! anybody hurt?”
“No’m.  Killed a nigger.”
“Well, it’s lucky; because sometimes people do get hurt.  Two years ago last Christmas your uncle Silas was coming up from Newrleans on the old Lally Rook, and she blowed out a cylinder-head and crippled a man.  And I think he died afterwards.  He was a Baptist.  Your uncle Silas knowed a family in Baton Rouge that knowed his people very well.  Yes, I remember now, he did die.  Mortification set in, and they had to amputate him. But it didn’t save him.  Yes, it was mortification—that was it.  He turned blue all over, and died in the hope of a glorious resurrection. They say he was a sight to look at.  


- B. Clay Shannon