There are various wood aging liquids like this one: https://www.rustoleum.com/product-catalog/consumer-brands/varathane/aged-wood-accelerator. Even though you say it's already aged, the sample I used it on gave an even gray-brown look to the wood. It's not a stain.

its the backsplash of a bathroom vanity, so i need a water resistant topcoat one i get the right color. Someone else suggested the dark parts are actually mildew in the wood and that i could scrub it with a brush and TSP to get it lighter.

here are some photos... first one is "before" during the day, the rest are "after" the weathered oak stain at night. There is Danish oil on the slab.

Nasty moldy, non kiln dried wood is not an appropriate backsplash. Burn that and start over.

That is ordinary construction grade lumber. You will not protect that to the level you think or really need as a backsplash. It is already turning crappy with the looks of mold on it already.

I don't think it's mold. All of my old wood around here has light and dark stuff in it.

I asked my contractor for barn aged rough sawn wood and this is what he used... I would hope its not mold. Its built in to the wall so I need to either clean it or live with it. How do I find out whether its moId vs aging? It wasn't that dark before I stained it, more of a greyish tone. Oyyyy. The slab at least is very nice.

just found this article online...https://www.connerindustries.com/is-it-really-mold-or-just-staining/ 

check this out at around 5:15. Im going to give this a go. If it cleans it then we will know its mold! https://www.youtube.com/watch?v=Y0Jb85Fi-6M

This also looks helpful for cleaning dirty/old/moldy wood. Going to do this on our outdoor fence. Oxiclean and TSP mix. https://www.chemistrystore.com/Make%20Your%20Own%20Deck%20Cleaner.pdf 

I think your project looks really cool. I also see the mold….BUT since I also like aged things, my suggestion is powerwash it, sun it, and use oil based stain and a nut oil finish (like all 9 layers) for water protecetion. But really, it is a lot more work to struggle with a piece of wood that you dont like, than it is to just got get a different one. If you use this wood, you’ll likely be replacing it in a year or two. Maybe, just cut your loss?

And pickling/washing is always some sort of pigment/acrylic that sits on top of the wood.

The nature of stain is that it penetrates the pores of the wood. To remove that you have to use something that will disloge and displace the stain to remove it, like a chemical stripper.

Good luck with your project. It looks fun.