I don't understand the "hard boil" v "simmer" talk. Boiling water is boiling water and never gets much above 212 whether its a hard boil, rolling boil or simmer does it? As such, a boil is a boil. Get it there and keep it there long enough to break shit down so it is easily removed.
i have done quite a few euros over the years and yes a boil or simmer will get the job done. But if you have your skull in a rolling boil for a long period of time teeth can start to pop out and the nasal bones will fall out. Nothing a little super glue cant fix. As kdm stated boiling also seems to stain the skull. Simmering seems to work much better in my experiences, teeth and bones stay put, and meat/cartilage seems easier to remove. Do you hard boil a pot roast to get it tender or simmer it lol.
Let me guess....green colored dish soap, ie Dawn. The blackish color is probably the green dye used in the dish soap. We use the clearest dish soap we can get. Usually an off brand. Then, DON'T boil the water if you can help it. Boiling tends to make the soup of soap, fat, lipids, blood, and whatever is in the water, stain the bone. I think it causes the fats, blood, and lipids in the skull to "cook" into the bone instead of emulsify and come out of the bone and into the water. We keep the water at around 190-200 tops when the skulls come out of the beetles. Hope this helps.