The Cornish Flag is a black background with a white cross. The flag of Cornwall is actually the the flag of St. Piran but it has been adopted as the Cornish National flag as St. Piran is the Patron Saint of Cornwall. Whilst having a certain popularity in Cornwall today, the flag does not appear to be more than a few hundred years old. Nobody has produced old manuscripts or paintings that date it any earlier than the 18th century.
One version of the legend of St Piran "finding" his flag is that the Romans had smelted tin in Cornwall, but the method of doing so had since been lost. St. Piran "rediscovered" tin-smelting by accident when the black hearthstone he was using happened to contain tin-bearing ore. The molten tin smelted out of the stone in the heat of the hearth and rose to the top in the form of a white cross. Hence the flag
One of the oldest crosses in Cornwall is the St. PiranCross which stands in the sandhills of Perranporth near the ruins of St Pirans Oratory. This cross is mentioned in a charter of King Edgar in A.D. 960, and was considered an old landmark even at that date.
An article in Encyclopædia Britannica is supposed to record that the flag was carried by the Cornish contigent at the Battle of Agincourt (1415). I have not been able to track down the original article, and it may well be web sites merely copying one another.
The first real reference to the St. Piran's cross flag dates from 1835. Davies Gilbert edited the history of over 200 Cornish parishes - "The Parochial History of Cornwall. Founded on the Manuscript Histories of Mr Hals and Mr Tonkin: with Additions and Various Appendices" ). Davies Gilbert was a national figure who had been President of the Royal Society 1827-1830, so was no back street journalist.
Under the parish of St. Piran-in-the-Sands he refers to "a white cross on a black ground [that] was formerly the banner of St. Perran and the Standard of Cornwall; probably with some allusion to the black ore and the white metal of tin." It is unclear where Gilbert obtained his information - perhaps from either oral tradition or maybe the "Manuscript Histories of Mr Hals and Mr Tonkin:"
It could be related to the Breton flag. The Kroaz Du (above) dates from the middle ages, probably just after the Crusades. It was given to Pierre Mauclerc by Pope Gregor IX in 1236 or 1237. The Kroaz Du was then used by Breton sailors and soldiers, and was probably the national flag of the independent state of Brittany until 1532. The Kroaz Du has been attested in reliable sources only since the XVth century. It is reproduced as the Breton civil ensign in the Almanach de Trodec (XVth century), the Manuel de Pilotage de Brouscon (XVth century), and on Homan's map (1559).
Cornish Flag - St Piran
Cornwall Historical Information Cornwall History page