The KBG probably won't choke out the fescue, but it will fill in bare spots. However, KBG spreads via rhizomes rather than stolons. Rhizomes should be below ground. You may want to take some of the weedy grass (including some roots) to your extension service to your extension service to get an accurate ID. I've seen blades of grass from the same plant get identified as two different grasses from pictures. It can be very difficult to ID grass from pictures. Many grass seed mixes have fescue and KBG in them. I think some types of fescue may mix better with KBG, but K31 grows much faster and has much wider blades, so I think a mixture of KBG and K31 fescue tends to look somewhat patchy. Where in WA are you? You may be better off seeding in the fall. It may not matter as much if you're in the Seattle area, since it rains so often there, but if you're inland where it gets hot and dry in the summer, a fall seeding will give the grass more time to establish before the hot dry weather hits.

There is no practical way to selectively remove these various grasses as your lawn matures into what Tom Cook at Oregon State terms a "climax lawn" of a mosaic of grass species. Chemicals that would affect the target would also affect your desired variety. You'll have to start with a clean slate if you want that even look. The link is to an excellent document that covers quite a bit about Northwest turf. Here is a link that might be useful: OSU extension

If it's the same stuff as what lines the roadsides in this area, it's Glandularia. Used to be included with Verbena, I think. If so, not native, but definitely ubiquitous! http://allthingsplants.com/plants/view/124328/South-American-Mock-Vervain-Glandularia-pulchella/

Yes! That has to be it. Thank you so much!