How do i finish my mossberg stock like this?

I assume you already have a wood stock.......

Remove the stock from the rifle..... Remove the rubber shoulder pad..... Sand it down with sand paper..... Then stain it with Ebony stain and partially wipe off most areas with a cloth rag..... Experiment on some other piece of wood to make sure you can give it the look you want......

http://www.dafitch.com/images/firearms/P2280003_01...

Look here how.......

http://www.youtube.com/watch?v=9OYMUKkcJyY

http://www.youtube.com/watch?v=oFh_AGspaEQ

Not going do do all that typing but if you want it black either paint or strip the finish off and use a ebony stain

Edit: I looked at Metropol picture and it looks like a dark walnut stain.The dark spots looks like the stain has soaked into the end grain