To my horror almost the same thing happed to me with the exact same weed. My weed issue started when I discovered what I believe to be patches of Annual Blue grass. From what I can tell the Annual Blue grass is seeding itself now because it will die once it gets hot, and return in the fall. So pre emergent needs to happen in late August beginning of September. I couldn't locate anything that would kill it, I read that its mostly controlled with pre emergent. It was in so few places I just started to pull it by hand. Then the weeds in your picture started showing up everywhere, and I mean everywhere. I've tried searching google images, the only one I see that resembles it is barnyard grass

If you already have St Augustine then you can recover it this season. The secret is to allow the grass to get up to 6 inches before mowing it back to 4 inches (mower's highest setting). The other secret is to ensure it gets water only infrequently but deeply every time. If you get an inch of rain, then you don't need to water. Watering frequency depends on temperature, soil, grass type, shade, cloud cover, etc. Generally this time of year it should be once a month. In the heat of summer it should be once a week. Water a full inch when you water. Measure that with a cat food or tuna can. If you are fertilizing with chemicals, then wait until the grass is growing and you have mowed it twice. If you are fertilizing with organics, you can start now and do it again in April.

Surely there is an appropriate herbicide for this application. It really seems to me you're missing the obvious, first by not bothering to read the label of whatever you killed your old lawn with, and now by assuming that if you don't get just the answer you seek on an online forum mostly consisting of amateurs, within the first 24 hrs. of posting your query, then that search is over! Wow. I am not an expert in FL turf care but there are many people who are. Why aren't you consulting them? And why are you even bothering with any of this if your real heart's desire is to have an almost completely fake landscape? As a person who is not an amateur, it is folks exactly like you who are ruining the legitimate use of pesticides via your irresponsibility. If I behaved as you have done in the past, I'd be breaking the law, let alone retaining my employment. Truly one of the saddest posts I've seen here in some time. Very generally, oldmanroot's come closest to the truth of the matter: In lawn care as in the management of any plant community, one seeks to achieve cultural conditions that suit the intended plant community, in your case, a lawn of palmetto grass. So, whatever moisture regime that plant likes, that's what you try to provide, and on down through the other parameters you can control. This will do 90% of the work of having a good patch. Then, if problems persist, you reach for more extreme measures. BTW, I'm sure that if I wanted to, I could use the same interwebs you're on and determine an appropriate herbicide choice for your situation. I estimate it would take me five minutes tops. +om

Yeah, that looks like good ol' Tall Fescue to me. It is very interesting (or stupid) to put Tall Fescue in a bag of mixed seed. The rule of thumb for TF is if you seed it, ONLY seed TF. It is a fantastic grass...perennial, deep roots are great for drought, can handle both shade and sun, and makes a very nice sod if seeded tight & thick. The bunches of grass might get bigger from here, but it doesn't spread aggressively. If you have bare spots in there, I'd seed with more to make a nice, uniform patch. It only looks like a grassy weed to you because it sounds like it is mixed with other stuff....which makes TF look bad because of its growth habit.