This is a beautiful cake! The figures are perfect. They are really reaching to find fault with an obviously great cake. If they didn't specify black, I wouldn't budge on your price. You gave them a bonus of an expensive item like Sugarvail.
It sounds like they just want to get some money back on their cake. I'm of an older generation and it was uncommon (at least in my experience) to ask for money back after the goods were used or eaten. There was always one in a crowd. We were embarrassed by the few people that did this sort of thing on purpose.
I don't know anything about the online review system but, as others have said, just reply professionally or see if the comments can be removed. It's too bad that this is part of doing business now a days. Don't let them bring you down- you are an artist!