After surgery I had a black area on my incision. I was told to use silver sulfadiazine on it. I?

If the area was black, it means there was necrosis which is dead tissue. Sometime surrounding tissue can become bad too or be getting bad but it is just not showing yet. It is hard to say. It didnt do you any favors that you were allergic to the cream but it may not be what caused the wound to separate. It is more likely that it was just the dead tissue. When this tissue sloughs off it always looks worse than it did when the blackness was there. You can almost think of it like an old fashioned ice bag. You know how an ice bag has a small cap when you view it from the top but there is a larger area under the cap and neck? These wounds can be like this. There is a small amount of tissue showing that is dead and blackened, but underneath there is a larger area of non-viable tissue we cant see and once all of this sloughs off you can have a very large area. This is good though because now all the dead tissue has sloughed off (hopefully) and by cleaning and packing, it can fill in from the bottom up with good viable tissue. Be patient, it can take some time for this to fill in. You dont want it to close quickly because if it closes quickly there can be pockets and these pockets can get infected again so you want slow granulation tissue from the bottom up.

Probably not. You had an area of infection and that will pop incisions. Unless the cream caused a large amount of swelling, the it would make it separate. Years ago i had an area of an incision open from infection.