A young or immature eel, or an eel that appears pale or blank in color.
From blank (pale, empty-looking) plus eel. This is a fairly rare or archaic term, combining the appearance descriptor with the type of creature. The exact origin and development are unclear.
Blankeel is a delightful example of how English speakers combined observable characteristics (paleness, youth) with creature names—it's the kind of practical descriptive naming you find in old fishing and naturalist texts.
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