于 is among the oldest grammar words in the script — on oracle bones it already marked place and direction, like a compressed 'at/in/to'. Its shape may show breath curving around a barrier: meaning that flows toward a location.
于是 'thereupon, so', 终于 'finally', 由于 'due to', 属于 'to belong to'. Formal comparisons use it: 大于 greater than, 小于 less than — every math class in China runs on 于. It's also a surname.
The little hinge that pins actions to places and times — AT, IN, SO.
青出于蓝 'blue comes from indigo (yet outshines it)' — the classical way to say the student surpassed the teacher, and every teacher's proudest complaint. The 于 is the 'from'.