Not sure if this one falls into your list of "all Bluegrass" but I was there last year and had a BLAST!
River City Roots FestivalMissoula, Montana
In the City's Downtown
This Event is FREE
(I think this is held the last weekend of August, but verify that)
You can also check out events that may be happening at Lolo Hot Springs. Several of us caught Railroad Earth there the weekend prior to RCRF, in Missoula.
(Southern Colorado)
Folk and BluegrassPagosa Springs, Colorado
First weekend of June
Four Corners Folk FestPagosa Springs, Colorado
Labor Day Weekend.
Both of these festivals are help atop Reservoir Hill in Pagosa and have wonderful late night picking jams in the camping areas. The only drawback is the lack of showers on site, but then again, there are hot springs at the bottom of the hill. (thanks again Cindy Lou!).
Have a great trip!
Auntie Hope