Can you post a picture of the whole plant?

Also:

1) What variety is the plant?
2) Is it just this plant?
3) Where in zone 10 exactly are you?

Thank you for the help and response. 1. Oh sorry I thought I listed tomato plant. It's generic. And was given to me free for winning a little game. It was a free 1 dollar pack of a few seeds. Just listed as cherry tomatoes. And I didn't think much of them to grow well but it ended up growing great haha. About 5 maybe 6 months old? 2. Its a patch of 7 vining tomato that are affected. Herbs about 10 ft. Away haven't been affected. 3. Just below the 9b line. Tampa, FL which is mid florida on the west coastline. High humidity, lows of 70s, highs of upper 80s. I'll post picture in 1 moment. Thanks again.

Is the dark coloration creeping up from the bottom of the plant?

Certainly it did. It was lower on the plant. But I'm pretty sure higher than the soil line. About the lower middle, About 1 or 2 feet above it. Starting with just 1 of them. Then 2, then 4. Now it's to the soil line and up half way all 7, creeping it's way higher. Hasn't affected the fruit. But all branch and stem are inflicted.

Started 1 or 2 feet above the soil line, that is.

It's currently low on foliage due to a few horn worms and little tiny green ones i think are infant horn worms. Ive had a couple horn worms before, but now I had 5 much smaller than last and many of the tiny ones. I went to war with them and removed any possible damage where they could be hiding. Here a photo of the tiny ones, if you could tell me what you think. But I am sure, it is unrelated to the creeping black stems.

Tomato

On the watermelon plant about 15 ft. Away I found significantly less of these than the tiny ones on the tomatoes. These are slightly larger. But I believe them to just be older than the tomato worms. As you see a tiny mites there aswell? I've been in full attack haha.

It may be a reaction to too much sun on the stems or something. But I'm concerned it may be russet mites which are, unfortunately, too small to see with the naked eye. They usually start low on the plant and creep up the stems, eventually taking over foliage and fruit. They leave a characteristic "bronzing" damage and the leaves they attack look like they dried out and have a crispy appearance. Take a strong magnifying glass (14x or higher) and examine the undersides of all the lower leaves.

Information on russet mites and photos of foliage damage: http://ipm.ifas.ufl.edu/resources/success_stories/T%26PGuide/pdfs/Chapter4/Russet_Mite.pdf

Photos of russet mite damage to stems & fruit:
https://kentuckypestnews.files.wordpress.com/2014/07/fig-2-tomato-russet-mite-dmage.jpg
https://kentuckypestnews.files.wordpress.com/2016/04/ht-mites-fig-2.jpg

As for your pest photos the top one is some kind of pinworm, which is a pest. Spraying BT or spinosad regularly will control them, but once they hole up inside the "pockets" they make in the leaves, they can be hard to kill.

The bottom photo looks to me like a hoverfly maggot. These are actually beneficial predators. Hoverfly maggots eat pests like aphids and other small sap-sucking creatures. There are many different species of hoverfly, and many of them are aphid predators. They're a very good thing to have in the garden. Here's a video: https://www.youtube.com/watch?v=xZruUlt8K8Q

Also, I think you have way too many plants in that one pot. But this is your first year growing tomatoes, and there's not much to be done now. Just for future reference, that pot is maybe large enough for one tomato plant.

Oh yes, the size of the pot is testament to my inexperience, but mainly disbelief these dollar store seeds would yield and grow so large and quickly at all. They were actually in a smaller pot before that one if you can believe that haha. But the pack did not even state if they were indeterminate. But they've produced consistently and sweet tasty cherries. During the time I planted them, i also planted several other types in different testing sites in the ground, bigger pots, etc. And somehow in every category, these have done the best, even in these conditions.

But I did notice some full sun issues so I mulch and placed under the over hang on porch so drastically reduce direct sun. Not too long after that is when I noticed the blackening issue. The mites are a new development along with the pin worms. To date I've only seen very few aphids and just a couple horn worms. The Pin worms definetly are troublesome but I've had much success by going through every leaf and removing any damage at all. I believe to have it nearly under control. To this point, the foliage and fruit seem to be doing fine. But I felt like some lower branches are becoming more brittle to breaking with the spreading.

I will follow your direction and see if I can figure out what's going in there with mites. The last photo I provided, you can see the largest mite I've managed to see on the watermelon, just above the worm. The last 2 photos are zoomed to the same extent. That large mite is followed with clearish specks that look like could be tiny version of it. I've been spraying for the worms and those mite looking creatures don't seem to move at all.So I think they've been decimated.

The worms are currently seemingly more troublesome. They've done damage to the leaves that leave honeydew and very similar to the tomato pin worm. Striping seems a little different and many times larger ofcourse. But same damage to my leaves. They don't embed inside the leaf as much at all like the pin worms, So I wondered it was just a big brother of sorts. I will do my research and see if I can go with the correct course of action.

Again you were a ton of help and have me in the right direction. Thank you so much for your time!

The "mite" in your photo is not a mite. It's an aphid.

The russet mites I was concerned about are, as I said, not visible to the naked eye. They're extremely tiny and can barely be seen at 14x magnification. IF they are on your plant. You have to examine the area of the stem and the undersides of leaves directly above the greasy bronzed areas with a strong magnifying glass to see the mites at all.

And, as stated, the second photo shows a hoverfly larva, probably in the process of hunting and eating the aphid in the photo. It's not a caterpillar at all, and definitely not a hornworm, which is a very large caterpillar:

Please make sure any pesticides you use have been approved for use on vegetables. If it doesn't say so explicitly on the label that it's OK for edibles, it could be too toxic for safe use.

Good luck.