In HSDS, Xiao Zhao used her own blood to reveal the hidden text on the QKDNY manual. This invisible ink technique is a cliche in wuxia fiction and movies, but is it based on any scientific principles or is it pure fiction rubbish? I know lemon juice can be used as photosensitive invisible ink... are there other known methods based on chemical reaction perhaps?
Also, besides using blood, are there any other methods used in wuxia fiction to reveal hidden text?