Actually the KBG is supposed to outcompete the fescue as it's a spreader and the fescue is a bunch grass that does not spread out by runners or other means. The problem with doing blends like this is you have to select varieties that look good together then have perfectly even germination to ensure you don't have pockets of like grass throughout the lawn.

Under ideal conditions, the KBG will spread and "win." If you have some shade, under-fertilize, don't water etc, the fescue might tolerate that a bit better and will be more dominant as the KBG thins. It won't spread to fill in the spaces though and you might end up with a more patchy look.

crabgrass
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tall fescue
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Awesome! Thanks for the photos. They totally help to see a side by side comparison. I'll print them out and then show my neighbor. Thanks for all the great responses. I learned a lot!

If you water once a week (an inch each time), the KBG will definitely outcompete the fescue as it spreads via rhizomes. If you water less often, the KBG will go dormant and you'll have a mostly brown lawn with clumps of green grass where the fescue is. Then, in the fall and spring, when there's enough water for the KBG, it'll spring back and fill in and outcompete the fescue.

If you want a grass that can handle very little water and spread through the lawn when the KBG is suffering, something like streambank or western wheatgrass (both natives) or Roadcrest or Ephraim crested wheatgrass (imported from Siberia) may do it. Note that most varieties of crested wheatgrass are bunch grasses. There may be others that are rhizomatous, but I know the two I listed are.

Buffalo grass will spread via stolons, but I think it would be a mistake to try to plant it into an existing KBG/Fescue lawn. Buffalo grass does best during the heat of the summer and will probably be dormant from late September through mid May (I'm assuming you're in the Salt Lake valley; if you're around St George, it'll have a much longer growing season).

If this is an irrigated established lawn, then it is a clumpy combination with both of all three grasses. All will co-xist.

If this is not watered, or worse, a new lawn, then with a drought, the grasses will go domant, and some will die. P. Rye and KBG will go first. T Fescue will be the last one remaining. After a severe drought, you get an almost T Fescue lawn.

Achang--The OP is in Utah. Without irrigation, I don't think any grasses would survive except for natives and a few introduced species, like crested wheatgrass and sheep fescue.