Discolored finger nails

I am seeking advise and information.  I will try and make this short.  My 7 year old daughter finger nails and toe nails are turning black and are seperating from the skin.  I took her to a determologist and he really could not come up with a diaginosis or reason why this is happening.  He ran some test of the nail and it came back negative for fungus.  It appear to be spreading to other finger nials (so far 3 finger nails).  He prescribed some cream to reduce the thickness and it appears to be working, although, keeps speading.  It is now starting to showing up in her toe nails (4 toes).   Doctor suggest that it might be eczema in the nail or some for of psoriasis in the nail, can this be true?  I would appreciate any answers on what this possible might be and/or any advise on what test or other doctor I can see to pinpoint a diagnosis?  Any information would be highly apprecited since I am really not getting anywhere with my current dermatoloty.  Thanks, MHutch