Lovely story in today's SF Chronicle about the successful reintroduction of endangered mission blue butterflies to San Francisco's Twin Peaks.
From personal experience there are a lot of "blue" butterflies and most of them are hard to identify. I've often spotted these small butterflies perched on plants with their wings stubbornly closed; in more than 10 years of trying I've only gotten a few semi-decent photos of blues with their wings open. Even with a great photo they are difficult to identify; the crucial markings can be quite subtle. In some cases you would need to get a very good look at the butterfly open and closed to differentiate.
If you care to go mission blue butterfly hunting (or like to see other more common butterflies including swallowtails and painted ladies), the best place is San Bruno Mountain, and this is the premier time of year for a hike there -- try the Summit Loop on a sunny day. Loads of wildflowers to see, and a chance to polish your butterfly ID skills. "Butterflies through Binoculars: the West" is a really helpful guide. Don't forget your camera!