 Its all in the tuning
If you want to run a blow off on a turbo or even supercharged car, you would need a stand alone, speed density type of engine management that uses a MAP sensor which measures manilfold vacuum, an AIT sensor that measure Air temp coming into the motore to measure how much air is coming into (car computer takes these three numbers along with predermined VE number (the part we adjust during tuning) to calculate how much air to add.
99% of cars today run a MAF sensor instead. which directly measures air mass going into the motor typically by a heated element, moving trap door or some other method. These systems are much more accurate than speed density as they adjust automatically to weather by design since they measure the amout of air directly and not indirectly like speed density hence the reson the OEM uses them. More accurate readings means less emissions and a smoother running car.
Back on topic
On a forced induction car. If the MAF sensor is in front of the Turbo/Blower as is the typical case then running a Blow off valve will trick the car to think it has more air than it really is injesting during shifts and light throttle when it is blowing off. This is because the blow off valve dumps air already measured by the maf sensor into the atmosphere, the car assumes its in the motor and adds extra fuel which causes this very rich fuel mixture. The car will sputter and spit a little black smoke out the tail pipe or even a backfire (running rich) between shifts or a mid throttle just as you go into boost. In extreme cases it can be so rich that it stalls the car.
Most aftermarket engine management systems use the speed density system to measure air because it is soo much cheaper and easier to install to any type of motor. A maf typically is very specific to a particular model motor and can't be easily adapted for many cars. You can use universal MAP and air temp sensors that can be attached very easily to just about any type of engine. MAP needs a vaccume line out of the intake manifold and a IAT sensor can be easily attached via a small hole in the intake tract of the car to measure incoming air temp.
If you ever seen an Mitsu evo at night with alot of boost, no Catalitic Converters and a blow off valve you may know what I'm talking about, you can even see flames shoot out the tailpipe at night . It might look and sound cool but they are paying with it via drivability. I had and tuned Evos, awsome car for the money! A poor man's porche
I have been tuning cars for quite some time now albiet not a Bimmer. If you have any questions about forced induction, tuning, or modifying cars in general Id love to try to help.
BTW u can get a greddy type RS blowoff valve without the horn attached and use it as a recirc valve for the blow off sound. Thats what we evo guys did | 
i like how you replied to jasons and not mine and i said the exact same thing first
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