Am I ready to stain?

You need to get the black out otherwise it will show. You would need to strip and brighten for removal. Other option is to use a very dark colored stain to blend.

We were already thinking of doing a dark stain, we thought about TWP 1503 Dark Oak, and TWP 1504 Black Walnut. Do you think those are dark enough?

Black Walnut for sure. Maybe on the Dark Oak.

Could I also just sand down the dark parts?

Striping would have better results than sanding them out. I would blend with a dark stain.

I'm not sure where to ask this so I hope this is right. How long do you wait between stripping the deck and then brightening it? Then how long do you wait before you can stain it? How do you know it is dry enough?

You apply a wood brightener right after the stripping is done to neutralize. Let dry for a couple of days is normal but this will vary based on the stain brand.

I have a 600 sq foot deck I am stripping that hasn't been treated in 8 years. There appears to be a few layers of a stain and a dark brown color stain. Since I can't do this project quickly should I strip a small portion let it dry then sand and brighten or wait a month to get it all stripped and finish one step at a time. I don't know how long it's going to take me to get it all stripped. I just hope I survive this:). I can't afford to pay someone to do it. Also, the deck is 24 years old and in good condition not sure what type of wood. It does have ridges in the railings and a ton of spindles to clean. What is the most durable stain I can use?

Strip all that you can, sand the rest off if needed, brighten all when done.

Look at the TWP or Armstrong Clark in a semi-solid color.