Actually in most Psilocybe mushrooms the mycelium does produce psilocybin, generally less than fruiting body but still significant (I just recently posted something related
here)
Also, the relationship between blue-bruising and psilocybin content is not exactly clear. While often it can be a good indication, its not necessarily related. For example check this quote:
Gartz et al 1994 wrote:Psilocybin was found to be present in the cultured, non-bluing mycelium of P. sumuiensis grown on 6% malt agar. Amounts of psilocybin, ranging from 0.24% to 0.32% dry weight, were analysed in 5 different batches of mycelium grown over a 4-week period.
On the other hand, apparently some Boletus as well as some Mycena mushrooms will bruise blue and not contain psilocybin (Voogelbreinder 2009)
Here's an interesting article on the relationship between bruising and psilocybin content, and a theory on why it may happen. In
Trout's Some Simple Tryptamines book he also discusses other hypothesis and proposed molecule structures for this phenomenon.