In 1337 King Edward III upgraded the existing earldom of Cornwall and made it into a duchy. He also established the convention that it would henceforth belong to the eldest son of the monarch. The recipient in 1337 and first Duke of Cornwall was the seven-year old Edward of Woodstock.
On coming of age young Edward ensured that Duchy offices were packed with his own men. Very few Cornish in the years before the 1460s held Duchy posts. The Duke was keen to make more money from his estate. In addition, unlike the immediately preceding earls, he was also prepared to order action against local gentry who overstepped the mark and took the law too frequently into their own hands. He curbed local hard men such as John Trevarthian and Sir John l’Ercedekne, while imprisoning his own Duchy steward in Launceston Castle in 1357 for misdemeanours.
But despite this oversight, Edward remained an absentee lord, only visiting Cornwall twice, for a couple of weeks in 1354 and over Christmas and the New Year in 1362-63. Each time he ventured only as far west as Restormel. His main interest became squeezing the surplus from the Duchy to pay for the wars he was busy fighting in France.
Aquitaine. Forget Cornwall” Edward III rewards the Black Prince
At the age of 16 Edward had played a prominent part in the victory at Crecy. Later, in 1356, he captured the French King at Poitiers and took him back to England. Becoming Prince of Aquitaine in 1362 Edward never returned to Cornwall after his visit late that year. In his absence over the Channel, oversight inevitably became looser and even more remote. Cornishmen began to pick up more Duchy offices, while endemic lawlessness and family feuding returned.
Meanwhile, the Duke was getting involved in the Castilian civil war. More battles were won there until he contracted dysentery in 1367. Recurrent bouts of illness pursued him through his final decade and he eventually died of dysentery at Canterbury in 1376, aged 46.
And why was he ‘black’? This may be a later designation as the first reference did not appear until the 1530s. It’s been suggested that it came from the colour of his shield or armour. Others insist it stems from his brutal reputation in Aquitaine, where he was not slow to put the French to the sword. However he obtained his sobriquet, this martial Duke seems to have treated his Duchy as a convenient cash cow rather than any more meaningful constitutional possession.
Cornish studies resources 
Plus ça change, plus c’est la même chose! I can’t think of a single benefit the Duke of Cornwall brings Cornwall. And the black prince also became prince of wales. What absurdities are visited upon us from centuries and centuries ago.
Regarding the black bit of the prince, couldn’t it also refer to a dark complexion? I looked a bit online and see that more effort has been put into linking black to his behaviour or to his armour as you say, but neither explanation seems convincingly linked to historical sources. At the same time the appellation was around from very early days. It seems plausible to me it could be linked to complexion as this is something that is always discussed, Obviously we have no image of him.
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There is some debate about the devolution of the title Duke of Cornwall. The original charter almost certainly intended the Duke of Cornwall would be the eldest son of the monarch however Lord Coke redefined it as the eldest son of the sovereign being heir to the throne. Just as an example if Charles dad died before having children Andrew, under the initial definition would not have been Duke of Cornwall. Under the later definition Andrew would have been Duke of Cornwall since he would have been the eldest son of the sovereign being heir to the throne. Prince Albert recognised the economic possibilities of the Duchy and he laid the foundations for the present success of the Duchy
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