Hi Erin, glad to see your post again but sorry to hear you have more problems--I understand, since we have a 50+ year old ranch style home too built on a slab and continually find little nit noi problems to deal with too. on your black spots, sounds like excessive moisture from below may be the cause and you certainly should put insulation under your floors with the moisture barrier on the floor side, Plus make sure it is well ventilated in your crawlspace.
Now about your sagging floor, I have copied your latest set of info. regarding it: "When we went through the house we really noticed a large slant in the hall. My dad said "nothing to worry about just have to tighten the joists" (along that line anyways) We got the house for 82 000.00 asking was 95. we didn't know the cause of the sag untill after. Know that i've ripped up the shag we notice alot more. there is about a 2.5 in slant about 3 feet from where the beam is. Its plywood subfloor with old 9x9 tiles. sorry it took so long to get back, been very busy, thank you for any help "
I still would suggest you get a professional foundation/floor leveling company to come in and give you an estimate of what is required to fix it and for how much---you might get several estimates to compare to see if they all have similar solutions/prices. Hope this helps. Others may offer additional advice. LJ