Just remember that not all "active" mushrooms bruise blue, and not all blue bruising mushrooms are psychotropic. As with many such myths there is a kernel of truth inside it, but not one solid or reliable enough to stake your health on. Most people associate the blue discoloration with psilocybin, which is a precursor to psilocin, but the reaction is only an oxidation of phenols and has nothing to do with any of the psychoactive chemicals people tend to be looking for when they poison themselves.
Active (and/or poisonous) mushroom species grow on everything from cow patties to hay and straw, the wood loving species like the Cyanescens and the Azurescens can be especially potent but have many poisonous look-alikes. The Amanita genus is particularly dangerous to play around with, some of them (like the Muscaria posted above) are just unpleasant, while the Phalloidieae section contains some of the most toxic mushrooms known to man. The "fool's mushroom" (or "destroying angel," if names are any indication...) is extremely dangerous because it can be mistaken for several common edible field mushrooms.
There is no single indicator of toxicity in the fungi kingdom; there is no color, shape, smell, size, or marking that can identify a mushroom as absolutely safe or absolutely unsafe to consume. If you're going hunting the little buggers, make sure you take a reputable and well illustrated field guide and USE IT. Don't just assume that if the mushroom you're picking is the same color and shape as the one you're looking at in the guide that it's the one you're looking for. Study it; check it against several samples; look at the gill pattern, stipe, and veil; take a spore print if you can. Believe it or not, the best way to positively identify a mushroom is by studying the shape and size of its' spores under a high power microscope.
Like Bogus said...it's something best left to people who know what they're doing.