Blondel was the minstrel who was a favourite of Richard I of England and who, according to legend, discovered him when he was imprisoned in a castle in Austria.

The facts are that Richard Cœur-de-Lion spent little of his short reign (1189-1199) in England, and took part in the Third Crusade but failed to recapture Jerusalem, though his chivalrous competition with the Muslim leader Saladin became itself the stuff of legend. Returning from the crusade he landed on the coast of the Adriatic and tried to cross Europe overland, but in 1192 was captured and imprisoned by Archduke Leopold V of Austria in the formidable Dürnstein Castle, 160 m above the town.

I am unclear from the various websites I've looked at how much of them is legend, or just uncertain fact, about this next part, but either the imprisonment was on behalf of Leopold's overlord, the German emperor Henry VI, or Henry found about it and ordered the King to be handed over. Either Leopold or Henry transferred Richard to another prison once his original location had been discovered.

Blondel de Nesle was a real person, a trouvère or troubadour, born around 1155, and his music was well known all over Europe. The legend says that he was a boon companion of Richard in their early days, and when the King failed to return from the crusades, Blondel travelled from castle to castle singing his songs under any where there might be prisoners. One day he was rewarded by King Richard's voice returning the familiar song.

Whether anything like this really happened, Richard was ransomed by Eleanor of Aquitaine his mother and by the people of England, literally with a king's ransom, the huge sum of 100 000 marks. Released in 1093, he arrived in England in 1094, only to return to the wars soon after.

Further reading: tells the legend at length, in a Victorian style, with a nice old picture. for another retelling, with the castle named as Trifels has a poem Blondels Lied by Johann Seidl (1804-1875) that was set to music by Schumann

Was er sang, das singt er wieder,
Wieder tönt es ihm zurück,
Süßes Echo klingt hernieder,
Keine Täuschung, sichres Glück!
Den er sucht auf seinen Bahnen,
Ach, sein König ruft ihm zu,
Nicht vergebens war sein Ahnen:
"Suche treu, so findest du!"

Not to be confused with Blondin the tightrope walker, though I always do and have to look them up.