It's probably more trouble that it's worth. Unless the leather in question is either A) Veg-tanned and NOT given an acrylic finish or B) Chrome-tanned and full-grain with no sealant the dye won't evenly penetrate (or potentially soak in at all) which will leave you with either a blotchy finish or dye dried on the suface, but which hasn't sunk into the leather and which will buff off at the slightest rub (on your kilt, for instance).

If you want a black sporran for very little cash, you can get one on eBay for ~$18US. It's a low quality piece, but at least it probably won't get dye all over your kilt.

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