Fluoran is a triarylmethane dye. It is the structural core of a variety of other dyes.Hatano, Y. "The chemistry of fluoran leuco dyes" in Chemistry and Applications of Leuco Dyes, Ed. R. Muthyala, Plenum Press 1997
These dyes have a variety of applications such as chemical stains (for example eosins) and in thermal paper. Black 305 is a common leuco dye product for thermal paper.
References