Once upon a time, in Wales, there was a band of sheep. They were white. They were fluffy. They were dangerous.
These sheep didn't have a sheepdog, and they wouldn't have listened to one if they did. They spent their time loitering near the sea cliffs, sharpening their hooves on rocks and biting the heads off of gull chicks. All the extra calcium gave them lovely strong bones and muscles, which they used to kick each other in the head.
One day, a large boat drifted towards the shore while some of the sheep were down on the beach scrounging seaweed and stepping on crabs. (The rest of the sheep were up on top of the cliffs, pushing rocks over to see if they could hit anyone.) Though it was riding quite low in the water, the sheep were intrigued, and when it drifted close enough, they grabbed the dangling lines and towed it in.
The boat was abandoned; the only sailors aboard were long dead of thirst, or in some cases stabbing. The sheep threw them overboard, because what can you do with a broken sailor? The hold had been pillaged, the hull badly damaged, and the rigging all torn about. The only things left aboard were the anchor, some barrels of tar, and a slim Japanese volume on the art of ninjutsu.
Interesting, thought the sheep.
They hauled the boat up onto the beach, emptied out the water, scraped off the barnacles, and mopped up the nasty filthy ick that grows on abandoned boats. They wove new woolen sails, improvised rope repairs from seaweed, and bullied some gulls into helping them get the lines back up (but ate their chicks afterwards anyway, because they were wholly untrustworthy sheep). They patched up the hull with the barrels of tar. They got entirely covered with the tar while patching, of course, because they're sheep and sheep are clumsy. It matted up their wool and made them black and shiny and sleek.
To pass the time while the hull was drying and the gulls were weeping, the sheep studied the book of ninjutsu. Fortunately, it had illustrations. Not illustrations of sheep, mind you, but they were fairly clever and utterly ruthless sheep, so they figured out how to adapt some of the nastier tricks.
Once the ship had been repaired, the sheep re-floated it, filled the hold with sod and gull eggs, and sailed off to strike terror into the hearts of the local fishermen, and any passing galleons full of gold they happened to run across.
And that, my dears, is why we call pirates and ninjas and other no-goodniks whose families disapprove of them black sheep.