This is not vitally important, but your black axxie may be Wildtype! Toothless came to me as black, and within a month, her tummy was white, her head was olive coloured and her entire body was covered in gold spots. She was sold in the marine shop as 'black.'
An easy way to tell is to look at your axxie's eyes. Do they have a shiny silvery blue ring around the iris? You have a Wildtype. Does it have pitch black or blue eyes, without a shiny iris? Melanoid.
It may also be that the colour is changing as the skin sheds. Sometimes this is normal (if you were sure of your tank parameters, it could be considered normal) and other times it is caused by high levels of nitrite or ammonia, or a very high PH (axxies are quite lenient with PH and can cope quite well between 7 - 8, but it's still better to keep a weather eye).
I had problems with cloudy water too and it could be leaving food or waste in there (mine was caused by bloodworm) or inadequate filtration. Another cause is adding new sand. Do any of these sound familiar? Stress Coat is great as, like layna said, it can help wounds to heal.
If you are really concerned, then I would suggest temporarily moving your axxie into a large container and filling it with dechlorinated aged water (i.e bottle some tap water, use Tap Safe or Stress Coat, and let it sit overnight). You can keep the axxie in there as normal BUT you will have to replace the water daily in order to stop the axxie getting ill (as you won't have a filter). The idea is to use the dechlorinated aged water daily to fill the tubs from the bottles, and then fill the bottles up immediately afterwards, dosing with more Tap Safe, and leaving it to sit. That way you have an instant supply of aged clean water to clean every day. You will also need to keep this large tub in a dark, cool place where no - one will knock it over. I have an emergency large food tub for this, as well as a breeder tank (a basic 3 gallon glass tank) for longer term occupants.
I know this sounds like an awful lot to do but I've done the same for a month while my other tank cycled and am doing it now for my new axxie. Unless we have your tank parameters we can't know how safe your tank is.
I hope this helps!
