Black hairy tongue (lingua villosa nigra) may be associated with the presence of chromogenic organisms (e.g., C. albicans) and the use of certain medications (e.g., doxycycline and bismuth). The pathophysiology is thought to be due to proliferation of the filiform papillae of the tongue, which stain black with porphyrin-producing chromogenic bacteria or yeast.
I take exception to the statement from New England Journal of Medicine, however, that this photo is "commonly associated" with Candida. I have treated thousands of C. albicans mouth infections ("thrush") and have never seen a condition that looked like this from Candida. The staining comes from the medications used with it.