In our tale it's not King Arthur who fights the black knight but one of his knights. The Lady of the Well is retold in The Mab as Luned and the Magic Ring. Luned is the wife of the Black Knight and she gifts our hero, Owain, a magic ring which makes him invisible and allows him to complete his quest.
This tale is the second of the three Welsh Romances contained in The Mabinogion, all found in Welsh but with strong links to the French Arthurian tales.
This site retells the story in a brief way, or if you want a lengthier read try this version.
Sadly there doesn't seem to be a good online reading of this particular story.
Luned is also the Lady of the Well, wells and springs were hugely important in pre-Christian Wales and that importance continued with churches often being built on the sites of those wells. Many places names feature references to this being a location where there was a well or spring. The image above is of Ffynnon Ddrewi, Ceredigion, where there are 3 separate wells, each is supposed to cure a different ailment.
St Winnifrede's Holy Well has been a place of pilgrimage since at least 1115, probably the oldest continually visited site of pilgrimage in Britain.
The fibre today is called Magic Ring, and sadly is one for which I don't have a huge amount of spare fibre. What there is available is in the shop now, and if you miss out on that, but feel the need for more pale purple in your life Lavender and Provence are both very similar colours.