The characters literally assemble to mean: “Indigo blue (青) emerges from (出) the indigo plant (于蓝).” The full original phrase adds the crucial conclusion: “…but is bluer than the plant” (而青于蓝, ér qīng yú lán). This literal, natural process of extraction and refinement is a perfect metaphor for how a student absorbs knowledge from a teacher and then develops it into something even greater.
The idiom originates from the writings of the philosopher Xunzi (荀子, c. 310–c. 238 BCE) in his work “Encouraging Learning” (《劝学篇》). The original quote is: “青,取之于蓝,而青于蓝” (qīng, qǔ zhī yú lán, ér qīng yú lán), meaning “Indigo is extracted from the indigo plant, but it is bluer than the indigo plant.” This concept is deeply embedded in Chinese cultural values:
Compared to the Western phrase “The student has become the master,” `青出于蓝` is less confrontational. The Western phrase can sometimes carry undertones of a challenge or takeover (think of its use in sci-fi or action films). In contrast, `青出于蓝` is almost always celebratory and emphasizes the teacher's pride and the natural, positive cycle of learning and growth. It's about honoring the source while celebrating the result.
`青出于蓝` is a formal and highly respected idiom. It's used to give a sincere and powerful compliment.
The connotation is overwhelmingly positive. It is a statement of praise made by a third party or, most powerfully, by the teacher/mentor themselves. It would be considered arrogant and inappropriate for a student to use this idiom to describe themselves.