There are many examples of the use of a black-handled knife in pre-Gardnerian Witchcraft practices. A number of these examples come to us from Ireland, and some of the evidence would suggest that knowledge of the use of such knives was widespread. On the 22nd of April 1895, the London Spectator (newspaper) related details of the murder of a Mrs Cleary in the County of Tipperary (Ireland). Mrs Cleary was burned to death by her husband, who was convinced she was in fact a fairy changeling that had been substituted for his wife. He and his neighbours believed they would be able to rescue the real Mrs Cleary from the fairies.
"Again, after the burning, many of the men of the locality sat up all night in a 'fort' (earth embankment of ancient Irish village,) armed with black-handled knives. These poor people thought that a fairy procession would pass by; that in its midst would be Mrs Cleary riding on a gray horse, and that if anyone rushed forward and cut her bonds with a black-handled knife, (a potent weapon against all evil spirits,) she would at once be restored to the world." 
This use against fairy folk was not restricted to Ireland. Wirt Sikes’ 1880 book British Goblins described the use of the black-handled knife for protection from Welsh fairies amongst the most effective amulets:
“The more worldly exorcisms, such as the production of a black-handled knife.”
In a footnote found in the story of Betty Sullivan published in the Dublin University Magazine of 1849, and described in more detail in the subsequent chapter on the Magick Circle, we find the following explanation for the use of the black-handled knife, once again linking it to the practices of magick a hundred years before Gardner’s writings. Interestingly it also links its use to ceremonies performed on All Hallows Eve, which is of course known by the name of Samhain to modern Wiccans:
"A black-handled knife is an indispensable instrument in performing certain rites, and we shall have occasion to describe its virtues by-and-by. It is employed in the ceremonial of Hallow-Eve, and also in the mystic ceremonies performed at the rising of the new moon, as well as in certain diabolic mysteries made use of to include love etc."
The black-handled knife turns up again and again in Irish folklore of the nineteenth century. In yet another legend, The Man who killed a Spirit recorded by Florence M’Carthy in 1843 for The United States Democratic Review, a wise woman by the name of Ould Molly gave instructions to a young lady called Mary which involved the use of a black-handled knife to help free her love, Tom Malloy from the possession of an evil spirit. Mary rewarded the wise woman with a generous gift of tea (which was a commodity of great value at the time). She obtained a black-handled knife and took it to the suffering Tom with instructions on how to use it against the spirit. Following the instructions, Tom struck the spirit through the heart once (and only once) whilst on holy ground. Tom succeeded in ridding himself of the spirit and was able to continue his life with Mary as his wife.
A modern dictionary of Hiberno-English refers to a black-handled knife as a Scian saying:
“…'Scian na coise duibhe' - a black-handled knife (regarded as a protection against the spirit world)…”
Extract from: Wicca: Magical Beginnings written by d’Este & Rankine, 2008 (Avalonia.) PB / Kindle @ https://amzn.to/3Ay4HJr. Shared here with the intention to inspire and inform the now and future generations interested in Wicca and other Pagan traditions inspired by it.