Rhys Roberts 1924 enghere the ‘black eye’ is compared to a mulberry because of its colour, the exaggeration lying in the quantity of mulberries suggested.
http://www2.hf.uio.no/common/apps/permlink/permlink.php?app=polyglotta&context=record&uid=d22d6921-1d51-11e7-ba66-0050569f23b2