They're both set in school, and deal with some family issues that don't usually get brought up much in BLs. Blueming is a bit more serious about it in how they deal with theirs but both are surprisingly realistic in how they approach the issues.

As a result, both series have a lot of heart in them through the sense of honesty in how both are written and how the plot progresses. Blueming is a little more bittersweet and serious than 21 days but still ends happily, plus they have more kisses and sweet moments between the couple.

Both shows also have a one sided enemies/rivals to lovers main pair with a prickly tsundere.