Loranthomitoura (Callophrys) spinetorum spinetorum
This rarely-encountered butterfly is distinguished by the bold white line on the reddish-brown undersides, and by the flashing blue of the dorsal side seen as it flies. It uses dwarf mistletoes that grow on pine trees as its host, and thus is only likely to be found in pine forests taking nectar at flowers. I got my first photos in June, 2017, after many attempts over many years (mostly in known locales in the San Gabriels or Lagunas). Of course, when I did finally see it, I was southeast of Big Bear where I was looking for something else, but there it was on a blooming ceanothus bush swarming with bees, moths and siva hairstreaks. You'd think it would be possible to confuse it with the latter; however, that dramatic white line against the deep reddish-brown ground color makes identification easy.
Dwarf mistletoe species in our area include Arceuthobium campylopodum and Arceuthobium divericatum. Spinetorum is said to fly from March to August in one or two broods, but June seems to be prime time for them in our mountains.