
OMG it just dawned on me... the titles of this manga and the sequel... It's all from Saeki's perspective.
In the first book, Saeki carried a torch for Wataru, but he never thought that his feelings would be reciprocated. That's why he could only admire Wataru from afar while he stands there in the midst of summer, shining brilliantly. Hence the name "You in the blue summer".
In the second book however, they're together now, that's why it's titled "In the blue summer, with you". I didn't think it was possible to love this manga even more, but here we are...
Good news everyone! The author just announced on twitter that she's planning to release a sequel to this manga, a second volume if you will. You can follow her and see for yourself at @furunagi on twitter. I'm so happy to see more of one of my favorite BL couples!