Ribbon!
I had exactly the same thing happen with my very first quilt, also QAYG, where It hadnt properly caught the reverse side..
I happened to have exactly the right colour of ribbon as the fabric so i appliqued it on top ... you'd never know it was there!
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You could use the same colour or a complimentary colour, maybe make afeature of it and do a couple of other blocks too ... No one will know!