I think that's what used to be the "Shiny Plain Brown" that indicated the book has no cover. Open the book folder (press 'O'), if the book has no cover.jpeg then I think that curlicue decorated blue one is displayed - I preferred the old shiny brown.
If you want the cover image that's in the the epub to be extracted into the book folder, select the EPUB format file in Metadata Edit formats box, then click the blue book cover icon (top left) - it's tooltip tells you what it does.
BR
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