I agree it looks a bit like K-31, but at the same time it doesn't. That color is blue green, which isn't typical of K-31. Perhaps it is just a photographic artifact. Also it looks like it has a prominent central vein, as the blades have a distinct fold to them, which isn't typical of tall fescue. Definitely not crabgrass, which is too bad, that's not so hard to deal with. Here's a grassy weed id key that might help. http://www.ipm.ucdavis.edu/TOOLS/TURF/PESTS/grasskey.html

It looks a lot like Poa Annua, but it could also be regular Kentucky Bluegrass. My KBG is flowering right now, which is consitant with your pictures (I'm in zone 7 as well) If you had Fescue sod before, it probably had Bluegrass in it(a lot of sod farmers use it as companion to tall fescue in the transition zone) and that's what you are seeing. If you sprayed roundup and killed everything, then sodded or seeded with bermuda, then it is Poa Annua. You could also have a next door neighbor ( or very nearby) with a Fescue/bluegrass lawn and contamination is very possible by seed. Anyway, if is Poa Annua, you don't have to do anything, it will die on its own with the arrival of summer and by natural causes. If is Kentucky Bluegrass however, it will persist in your climate. If you really have bluegrass and don't want it, you can wait until Fall when your bermuda goes dormant, then spray roundup on it. That will take out the Bluegrass, but will not harm the Bermuda. More information than you probably needed.

We really need a picture to identify the unknown grass. I really do not think it can Saint Augustine unless: You have shade issues. You are not mowing, feeding, and watering per Bermuda maintenance schedules. Otherwise SA is not capable of overtaking Bermuda

Tony, what is the name of the pre-emergent you used? Is it a weed & feed fertilizer type? Since you said that that you applied two rounds of pre- emergent, I am of the humble opinion that either it was applied incorrectly or the product you applied did not work at all. I use a pre-emergent called "Prodiamine WSG65" with excellent results. WSG stands for water soluble granules. It is a very effective product and the efficacy lasts for up to 6 months. So if this product is applied in early May, you get the protection until November - by the time the window for crab grass seed / or any seed germination is past. Crab grass seeds can be viable for up to seven years and will lie dormant until the right conditions for germination arrive. If you don't keep up with the timely pre-emergent application, they are certain to come back. Apart from pre-emergent application, mowing high & letting the grass clippings back into the lawn, fertilizing the grass as needed etc. will also help. The whole idea is to not provide the favorable conditions for weed seed germination. Hope this info helps. I have fought this battle and won. I have very few weeds in my one acre lawn. This forum has helped me a lot in winning this battle. A big thanks to all the active posters in this forum.