I have recieved 99% of my gold (roughly 7 million) from selling sigils/items from HoH. Back when there were still normal chest/wreckage's in the game, there were spots you could just run to and recieve 200-400 gold per run. Each run would take only a few minutes. From what I recall, the average rate per hour was ~8k-12k an hour depending on the location and your class/skill combination.
I haven't PvE'd in the last 3 months, so I really don't know what the current gold farming spots are.
L

it is random on the black dye so there is no specific place to farm it.