There are a lot of theories about the blacking that people used to use on their staves.
Some people talk about rubbing lead on a staff, others talk about boot polish… Both are possible, this was a time when your house wasn’t made of treated timber, so the presence of borer larvae was a real thing for most people. If you had a favourite stave treating it to be resistant to water or borer was probably a reasonably sensible idea.
Some stories talk about rubbing crows or magpies blood on a stave, although we can’t for the life of us think of why.
What was probably the most common form of blacking was most likely accidental. One common ingredient that would have been available to waterproof a stave (and maaaybe help keep it supple) was butter. By rubbing a stave in butter it would be possible to treat it against water. And as staves were often conditioned in the flue of a chimney, fine carbon from soot or smoke might well seep into the wood with the butter fat (or dissolve into the fat) and create a progressively darker wood over time (though never as black as todays’ layer-of-black-epoxy look).
At the Claymore workshop we still like to use both blacked staves, and natural clear varnish.
If anyone asks why we still make clear varnished staves we’ll probably say its because we like to be old fashioned.
In practice, whilst blacked staves look stylish, natural wood grain and colours are something we think should be shown off where we can, even if blacked staves create a distinctive “shillelagh” style look.