Downtown Portland's Historic Postal Building, at SW 3rd & Washington, has a blank wall on the back side of the building facing 2nd. There may have been an adjacent building at one point, but right now it's home to a surface parking lot, and since the blank wall faces traffic coming off the Morrison Bridge, it hosts a couple of very large billboards. Look below those billboards, though, and you'll see a blue butterfly that someone's painted. Google Street View indicates it's been there since at least April 2014, and it's obvious that graffiti near it has been painted over while it's been left alone, so I imagine it's supposed to be here. It's not signed, though, and I haven't come across anything at all to share about it.
If I had to guess, I'd guess it's supposed to be a Fender's blue butterfly, a local endangered species. The butterfly in turn relies on one rare subspecies of lupine, which is threatened by invasive species and habitat loss. This is all just speculation on my part though.