I read on here that several people were looking for a cure for black tongue - in my case this occurred after aggressive antibiotic treatment including metronidazole which some people do claim causes this horrible side effect, (amongst many! ). I suspected candida being involved and based my approach on that. The colour was similar to liquorice stain and involved the whole top of the tongue. There was no way to clean it off and I felt disgusting with it. The blackness has only just cleared away, it took 4 weeks, and I don't know which part of my regimen played the biggest part - if any- but here's what I did:
Diet very low sugar and carb, mostly veg really but with small amounts of fish and eggs. No wheat in any form. No dairy except for live yoghourt, plain, which I find quite difficult to eat but easier when grated garlic and turmeric mixed in - as a kind of 'dressing' for the salady diet. Celery seemed very 'cleansing' and healing.
I took acidophilus capsules at every meal. I also did a tongue clean after every meal - it was quite unpleasant but seemed useful, ACV on a piece of towelling (to be burnt or thrown away after single use - cut up clean old towel for this) wiped over the tongue as thoroughly as bearable, followed by a rinse with warm water. I also took ACV twice a day. I changed to a bicarb toothpaste and after cleaning mouth and tongue each morning and night I soothed the mouth with coconut oil - swill for as long as possible then spit out. It has taken a month but my tongue is pink again now.
Hope this works for others.