Actually, think of a system without bov. when the throttle plate is closed, the compressor stops, therefore no air is being sucked in through the afm/maf. No(very little) air is going into the engine, so fuel is close to what it should be.
Now put a BOV on. the bov vents the air out from behind turbo, so the compressor wheel is still spinning, still sucking in air from in front of the turbo. If the bov is not routed in front of the turbo, that air will have to come through afm/maf. The afm/maf will register air flow (therfore the ecu thinks there's air) and put in fuel, but the engine is not sucking much air in. There is your overly rich mixture. A BOV routed before the turbo essentially acts as a closed recirculation system, where the outlet of the turbo is going back into the inlet, independent of the TB and afm/maf (to an extent). Not much air into engine, not much air going through afm/maf. Close to correct fueling.
HTH
Christian