Fishing on Boulder Mountain UT should be good from right now until about the beginning of the second week of September, or about Sept. 6-9 or thereabouts. Actually, the fishing should be good for awhile after that, but the weather might be something else again.
My last trip up there around Blind Lake (elev. about 11,000) was between Sept. 7-14 2016, and for five days it rained, sleeted, snowed, and hailed. Constantly. The previous year was a few days earlier, and it rained, sleeted, and snowed for four out of seven days.
When it wasn't doing that, the fishing was pretty good. Though the dry fly bite was sketchy after all of that cold precip.
My best trips up there have been in and around the last week of August-first week of Sept. It will probably rain some, but probably it won't snow. And the fishing was always pretty good, sometimes close to epic around then.
Also, if you do head up to the Blind Lake area, after the first week of Sept., take a good 4x4 vehicle, a come-along, some tow straps, and a chainsaw.
Between the high elev. stealing your horsepower, the incredibly slick mud when it rains, and the likelihood of fallen trees across your roadways, those things become necessities if you want to get out of wherever you went in.
Good luck.