In the English language, black sheep is an idiom used to describe an odd or disreputable member of a group, especially within a family.
The term stems from a genetic effect in white sheep whereby a recessive gene occasionally manifests in the birth of a sheep with black rather than white coloring; these sheep stand out in the flock.
The term has typically been given negative implications, implying waywardness.
It derived from the atypical and unwanted presence of other black individuals in flocks of white sheep.
I'm not sure that this works in Iceland as about 50% of the sheep appear to be black.
The fleece of the Icelandic sheep is dual-coated and comes in white as well as a variety of other colors, including a range of browns, grays, and blacks.
Reference: Wikipedia