The "Black Moon" in this episode likely references a total lunar eclipse that occurred on February 14th 1794 and would have been visible from Cornwall at around 10.00 at night.
A so-called black moon actually occurs at the period of a new moon, when the moon is more or less invisible from Earth. It usually refers to a second new moon within a month; typically this occurs roughly every 32 months. This is distinct from a Lunar eclipse, which occurs at a full moon and is caused by the Earth coming between the sun and moon, and casting a reddening shadow on the moon.
Ellise Chappell (Morwenna Whitworth) & Mark Frost (Tom Carne) also worked together on New Blood (2016) as Wendy Roberts & Samuel Armstrong respectively.