Apples and bananas. Leave Blackguards their bull's strength, if they got approved they deserve it and the class is shooting yourself in the foot from a mechanics standpoint - a far cry from making a pact with evil entities for the sake of power.
No stacking detection spells because screw bards! While I don't think that stealthers need an easier time (acquiring gear is sort of the life of a rogue and becoming a ghostly ninja legend is something that should take time and dedication to achieve), I don't think detectors need an easier time either. There's a plethora of easily-found items to help detectors do their job, they don't need any more advantages.
Brap brap brap, balance knowledge up in this thread!