This is a tricky one to identify. It looks a lot like the summer azure, but emerges around the end of May (ie. earlier than the summer azure). Some say that a fresh azure with a pale underside found around this time of year is probably a Cherry Gall azure, but the only way to be sure is to find it laying eggs on Cherry galls ( or to raise it up from caterpillars found feeding on Cherry galls ).
I'm not 100% sure of either of these images. The one at left fits in terms of appearance and timing, but I'm not sure where the nearest Cherry tree was located. The one at right is somewhat worn, and appears darker and more blue than expected, but it was observed on Cherry Galls. Hopefully, I'll get some unambiguous shots next season!