I loved Paula Brackston's "The Winter Witch".
This short story references a blue well and the main character of that novel finds it and uses it to become a powerful witch. This prequel discusses the first witches who came upon the blue well and one witch in particular that harnesses it's power.
The main character in this story is Ceri who has been told by her grandmother that she has magic in her blood. A pestilence causes her to lose her entire family except for her sister. Fleeing the sickness the two head towards the ocean and come across the blue well.
This story clocks in at 69 pages and also includes some of Brackston's thoughts on writing "The Winter Witch". I did like the short story and only wish that this could have been expanded since I became attached to the character of Ceri.