[Arran]

 I tried phosphoric acid to remove black stains in walnut veneer and it seems to work, i brush it on the area affected, leave it there until the stain fades, then wipe it off with a damp paper towel or rag, then dry it with a hairdryer. Now someone on the alternative forum didn't think this was a good idea but it worked for me, I guess the stains are iron based since the magic ingredient in rust remover took them off. With a black mark as large as the one on the second cabinet what have to got to loose, it's either try some chemicals to re-veneer the cabinet.
Regards
Arran

P.S Wood bleach usually consists of oxalic acid, though there are other types of wood bleach including chlorine or peroxide.