A white spot on a print is better than a dark spot.

Retouching negatives is always tricky, and I wouldn't do it with a crayon, but it used to be a very common practice to retouch with crocein scarlet. In fact, this material was used for extensive manipulation of tonal qualities (see Lootens on Photographic Enlarging and Print Quality). I would think a crayon would be terribly inexact, but who knows?