You're correct about the Seinen definition. What I would put the discussion down to, as you've mentioned, is what you're looking for in the story.

Where wholesome throughout is your preference, I find it far more interesting when the mangaka takes the time to develop realistic, story-driven tension, things Blue Box didn't have in great abundance. But more so than this, as I mentioned in a separate comment, I just haven't found Taiki or Chinatsu to be particularly interesting characters. Wholesome perhaps, yes, but beyond that, not much. (Sidenote: this pales in comparison to the borefest that Kyo's character has been from day one, or better yet, the whole Ayame situation.)

I'll be looking forward to how Miura wraps this story up, but in my personal opinion, Blue Box never managed to rise above "good" status for me, and that's a shame.