Leather dyes

[Medicus matt]

Does anyone have definitive examples of the use of "black" leather goods

Yes.

and if so, what was used for a dye?

Pliny and Dioscorides both talk about using vitriol to make "atramentum sutorium", a black stain used by shoemakers.
Adding oak galls would perhaps make atramentum librarium but would the tannin from the oak galls really add anything when working with skins that'd already been tanned with oak bark, since the iron in the vitriol reacts with the tannic acid to create the black iron tannate?

When I've played with this, I've ended up with colours ranging from grey/brown to black, depending on the iron content of the vitriol.

You can replicate the results by using iron and vinegar to create a 'safe' vitriol (safer than mucking about with sulphuric acid anyway) which, when used on an oak tanned leather (which is dark brown, rather than the majority of very pale modern veg-tanned leathers, which, having been bleached, are probably a poor starting point for any attempt) gives a dark grey which, when oiled with neetsfoot oil, gives a nice black finish.