all i can remember is that the seme and the uke have a subtle master/servant relationship which even other people acknowledge. the uke has black hair and the seme is blonde, if i recall correctly his hair is long. they're in a science/astronomy club and the seme takes cares of his senpai uke because the uke neglects his own health and wellbeing

The first manga that crossed my mind is Koisuru Boukun, but the one who blonde (brown) long haired is the uke, not the seme. And yup, the seme takes care the uke.

Then it also remind me of Takaramono wa Hako no Naka, this one, the seme indeed has blonde(brown) long hair & uke is black haired, but the seme did not take care the uke, bc uke pretty capable by himself plus uke can be quite dominant

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that's not it > < but thanks!