New Build Grass Mud Patch

Hi, we have been in our new build house for around 6 months, the rear turf was done last week and is looking good. However, our front lawn is very different and was done before we moved in. For the most part it's okay but there is one patch is VERY watery and has been for a long time.

It worries me because there hasn't been too much rain recently yet it is absolutely soaked with some standing water. Seeing as the grass is virtually gone I got a fork in and hit something solid i'd say a good 6-8 inches down.

My worry about it is there is a drainage system very close (see pic) so it feels like all the water is coming up from under this patch, maybe from the drainage by the foundation or something? There is also really odd black stuff on the grass which I'm guessing is the grass rotting away because of the water?

Any advice would be great to revitalise it or we may need to get this bit redone? We have asked the builder to tell us what's underneath because it's rock solid and doesn't run the full length of the wall. It's already killed the grass and the path is right next to the front door.