Yeah,
after the throttle body but before the supercharger. That way the accessories you are relocating will have vacuum at idol and atmospheric pressure at wide open throttle (which would be full boost into the engine). This would be the same situation as before you fit the supercharger - vacuum at idol and atmospheric pressure at WOT for these accessories.
It would be a good idea to refit the idol control as it is a great thing. Things like the crankcase breather can be deleted and replaced with an oil catch can, which has some advantages, but may give you some dramas for a roadworthy certificate of emissions testing.
You would put your blow off valve after the supercharger and before the air enters the engine (of course!!), but I'm not sure why you would want to fit a blow off valve??
To regulate the boost pressure on a positive displacement supercharger, just change the pulley sizes untill you get the boost you want. You don't need a blow off valve.
I'm not a turbo freak, buit I think they are intended for turbo setups to allow excess boost to vent as you back off the throttle (especially during gear changes). By venting the excess air, the turbo dosen't loose it's rotational speed and is spinning fast enough to have boost come on quicker when you plant your foot again after changing gears.
I have heard of some people have used them on cyntrifugal supercharger setups so they can run a higher drive ratio and bring on boost in the lower RPM range.
At higher RPM, the high drive ratio creates too much boost, so the blow off valve vents excess pressure to regulate the boost to a level that the engine can handle.
A bit of a waste really, because the supercharger is drawing excess power to make the boost and then it is being wasted as the excess boost is being bled off anyway. This is the tradeoff to get a cyntrifugal to create boost in the lower RPM range.
The only other reason to fit a blow off valve is for 'wank' factor.
