This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 靘 is uncertain. It visually combines 青 and 色, but the original semantic and phonetic roles are unclear.
With 青 (blue/green) and 色 (color), imagine a deep, pure color that is hard to name—fitting for a kanji with no clear meaning.
For セイ, think of the clear blue sky (青) as a serene (セイ) color. The reading セイ echoes the calmness of the color.
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