Steve
Enrich n seal usually works pretty good-its possible your bluestone has something blocking the color enhancer as Carl stated
Do a water test and see if it is absorbing.
Bluestone is quite porous and should darken considerably.
The color enhancers I like and use are:
Stone glamour or sigma seal from sparks- dark effect-works well outside.
intensifia from dry treat- dark effect
MB-21 Ultimate -dark effect
SCC color enhancer-
Just a note-I don't like to seal bluestones mainly due to poor installations and northeast weather.