This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蔧 combines the grass radical 艹 with 彗, which may have contributed sound. The exact historical development is uncertain.
The grass radical 艹 sits above 彗, suggesting a plant. Without a clear meaning, picture a mysterious, broom-like plant named after its sweeping shape.
For セイ, imagine a sage (セイ) using this plant in a ritual: the sage's name cues セイ, and the plant is part of the ceremony.