Right, here goes........
In the early 1800's scurvy was rife amongst seamen, and one of the symptoms was a darkening of the extremities.
This gave rise to the name of one of the most famous of all shipping companies.
When James. S. Baines wished to run his fleet of packets ships on the N.Atlantic trade, he wanted only prime sailors aboard his vessels, none of "yer lubbers" as sailed with his competitors.
So he sent a message to all his captains to only sign seamen who have "had the scurvy" and were, therefore, old salts.
He further told his captains that the best way to decide whether a sailor had ever had the complaint, was to check for a "darkening of extremities, in particular, discolorisation of the genitals" as limbs tended to revert to a more normal hue.
This, of course, gave rise to "The Blackball Line".
Boom Boom.