Hi again.
I think the image shows a bird with darker than normal plumage for the subspecies insularum (which lives in the eastern islands of the Canaries, including Fuerteventura) due to oil or other substances impregnation and not as a case of melanism or similar. We know another recent case from the same locality, and after examination of the images, we concluded the bird was impregned by oil or similar products. The fact is that the species frequently visits pig or chicken farms in our islands, where the birds can contact with different types of substances that produce a temporal alteration of the coloration.
On the other hand, I don't believe on Fuerteventura or Lanzarote there are individuals of the subspecies distinctus, which is endemic to the central and western islands of the Canaries (Gran Canaria, Tenerife, La Gomera, La Palma and El Hierro). The differences in plumage between these subspecies are not so evident in the field, but morphology and genetics agree to support the validity of both taxa.
Best regards from the Canaries.