It has one of my favorite tropes: best friend falling in love with each other, and there are plenty of manga like this, but so far, Kiss Blue is one of my favorites in depicting this kind of relationship.
This manga focus on two college men: Tomosaka and Noda. Tomosaka has been in love with his best friend Noda for a long time, but he doesn't dare to confess because he doesn't want to lose the friendship. It doesn't help that Noda jumps from woman to woman, even so far as two-timing or three timing.
Sure, it sounds very typical yaoi manga, but what makes this manga special is the character-building. Personalities are deep (beyond the blushing-in-every-panel kind of thing), and the characters' voices are relatable: Tomosaka, Noda, and the cafe owner.
The slow budding romantic relationship feels realistic, especially the friendship. Both boys do not want to hurt the other intentionally and the communication problem is addressed, not at the spur of the moment, but when one character makes a conscious decision that he has to communicate his feelings.
What I also like is the fact that Tomosaka and the cafe owner do not hook up and use each other. Tomosaka makes it clear that he does not settle for less, and he won't get into another relationship when he still has feelings for Noda.
Finally: the art. There is a sketchy feel to it, but the art is clean and simple; the soft lines give a melancholic, peaceful atmosphere. Every character is nicely drawn. The art matches well with the mood of the manga, which I really enjoy. Paneling is done beautifully.
All the characters are mature, and all the relationships well-built. Definitely a good read for readers who enjoy a slow-paced manga.