There is a few deck cleaners out there you can try. Most common that most people use are TSP cleaner, you can get it in most box stores.

They also have other cleaners like Restore-A-Deck also does a good job, you can see if you can find Penofin that's more of a pro-grade cleaner not sure how it works on already stained deck, maybe if you add more water to it to make it weaker it will do the job.

Just read the manufacturer directions and try to determine what the dark area is i.e dirt, mold, etc and make sure the cleaner you select intended for that type of a staining.

Most pros use their own mixes using regular household cleaners, or oxidizing bleaching solvents, etc.

Whatever you decide on, just make sure the cleaner you select is also safe for surrounding area like landscaping, grass, etc so the run-off will not harm anything.

Good luck

Just cleaned our deck today with straight white vinegar and a power wash...

This deck is quite old and the previous owners painted solid stain over mold and mildew...really smart, and now it's all peeling off...what a mess!

I sprayed straight white vinegar all over the deck, including the railings, posts, floor, etc. Let it sit for about an hour, then power washed...worked pretty good, but yes, it still has a lot of the old stain, but it did take off the mold and mildew...I will still need to re-do a few spots and use some scrubbing with a brush and some scraping.....lots of work..

I plan on painting it with a fence paint for the railings and porch floor paint to cover up the old stain...don't know if it'll work, but it'll look better than it does today, I'm sure....

I did look on line to see homemade recipes for deck wash, but thought I'd try the vinegar first, as it's not too harsh on the surrounding plants, shrubs, etc. I think it worked pretty good...but we'll see...

I don't know if the vinegar would take up your dark spots or not, but it's worth a try and won't cost you a ton of money.

Good luck with your deck...