The older border use concerned payments demanded in return for protection from plunder. The mail element meant payment or rent.
The puzzle does not assign a particular historical reason for black. A threatening black envelope is not the source story.
Sources
- Merriam-Webster: blackmail
The compound is Scots and northern English black plus the financial mail word.
- Merriam-Webster: mail
The older financial noun meant payment or rent.
- Merriam-Webster: is-blackmail-from-mail-that-is-black
The early border payments bought immunity from plunder; postal mail is a different word.
- Online Etymology Dictionary · blackmail
Meanings of the older forms on the word path.