KBG will look brown under heat stress. When you say you live in NoCal, where is that exactly? If you tell me you live in Sac'to or the Bay Area, I would argue that you live in north Central Cal. It is kind of important. If you live in Redding or to the north or west, or up in the mountains, that is different, too.

Fescue will never fill in anything. Fescue is a bunch type grass that just sits there for practically decades before enlarging the one plant. The idea with planting fescue is to get enough plants that when they grow to their diameter of 1 inch (max) that you have enough plants to become dense. KBG on the other hand will form a dense blanket and fill in anywhere a fescue plant dies.

Did they roll the sod down with a water filled roller? If not it is not too late to do that. The sod must have contact with the underlying soil. Roots will not grow through the air to reach the soil underneath.

You should be watering 3x per day for about 10 minutes each time, until the roots knit into the underlying soil. Then you can start to deep water. Deep watering means you soak the soil down deep by applying 1 full inch of water all at one time. Then allow the soil to dry out at the surface for about a week (this time of year - and depending on where you are in NoCal). Then water again a full inch.

Set your mower to the highest setting or one notch down. Short grass gets much more heat and drought stress than tall grass. In fact you can set it all the way up and go 2 weeks between mowing if you want.

Are you going to fertilize with organics or with chemicals?

You can have a spectacular lawn back there if you water, mow, and fertilize properly. And if you are doing that, then you will hardly ever need herbicide, insecticide, or fungicide.