I tried to dye some leather boots a few years ago, not riding boots, they looked OK to start with but then when the leather cracked a bit the old colour showed through. You may be able to get it done professionally I know you can have shoes done for weddings etc but not sure if that's just fabric ones, then again the cost may not be worth it and you may just be better getting the black ones, also with riding if the dye doesn't penetrate the leather enough the old colour would prob rub off on the horse!
