It's not entirely certain that this is what causes the blue bruising reaction...

the bruising may be due to psilocin oxidation to a 0-quinone by-product which is a dark blue color, or some say it is caused by psilocybin dephosphorlating to psilocin...

However

Many mushrooms blue that do not contain psilocin, Boletes and ploypores bruise blue a result of the oxidation of pulvinic acid derivatives, like variegatic, xerocomic, and atrotomentinic acid. Lactarius indigo is blue due to the presence of a compound called (7-isopropenyl-4-methylazulen-1-yl)methyl stearate. Entoloma hochstetteri gets its blue color from Azulene (bicyclo[5.3.0]decapentaene)...

Also, there are many species that contain psilocybin which do NOT bruise blue.

if the species you are seeking or that you think you have is known to blue, than this reaction is crucial to look for...

...but just going around looking for random mushrooms that turn blue in search for psilocybin is completely incorrect.

-eg