I would like to use black in my player sprites, how can I remap the color so that, for instance, color 12 is transparent in that sprite instead of black?
Thanks.

Black is always transparent, so you can't make it opaque at all in Pico-8 currently.

So for my game I changed that instead of black, it uses the beige (color 15) as the transparency. So to have that for the whole game I used this:

function _init()
	cls()

	palt(15, true) -- beige color as transparency is true
	palt(0, false) -- black color as transparency is false
end

But if you only want to change the colors of one sprite itself:

function _draw()
	pal(8,8)    -- here I set red to be red, since I changed it on the other sprite below
	pal(2,2)    -- same here, keeping my purple, purple
	spr(1,40,50)  -- normal sprite drawing
	rst_pal()  -- resetting color pallet

	pal(8,9)	-- changing red to be orange
	pal(2,4)    -- changing purple to be brown
	spr(1,50,50)  -- normal sprite drawing
	rst_pal()   --resetting color pallet
end

-- since I changed my standard color pallet for the whole game I use this function.
Otherwise just use pal() instead of my custom rst_pal() function

function rst_pal()
	palt(15, true)
	palt(0, false)
end

There might be other ways to do this neater, but at least this works for me! I hope this helps :)

Edit: This means my sprites look like this:

The black stays black, and the beige becomes transparent

With pal and palt you have all control over color and transparency in pico8, you can even use them to fade sprites in and out.