I would wash out whatever is dissolved in your black mess, and sort out any material that didn't dissolve into solution, which could be gold foil or almost anything really, who knows what your pins were made out of, do you know?
rewalston is right though, warm HCl then sulfuric cell, at least that's how I do it. You should think of your process as being two separate processes. First, you recover and second, you refine. Only use AR to refine unless the type of material you are processing requires you to use AR before another recover process. You will save yourself the mess.
When I process pins, I use a sulfuric cell to recover the gold, I also put the Pyrex cell in another dish that has ice so I can keep it nice and cool, so long as the Sulfuric Acid is cool, it will only deplate the gold alloy from the base metal. Once it gets hot however, it will eat just about anything you put into it. You don't want that, so if you use a sulfuric cell, make sure you keep it cool, and if you have a lot of pins, I would be careful not to do too much at one time because you will create too much amp draw from your power supply and will probably blow a fuse. Using a regular sears 12v 10amp charger I do about half a pound at a time, it seems to work best.
There is a ton of information on the forum on this subject, you can search and find everything you need.
As for the mess you have made, you should read Hoke and what she says to do about waste, then reprocess the part that has any gold. I would also test your solution to see if there is any gold. Also, did you dissolve your pins in plastic? Or where they totally clean pins? The black color could be from plastic, specially if it's really thick.
