As Bubba said, it wont run rich unless the BOV is actuating.
If the BOV is actuating under load. Its not tight enough.
Properly working Atmospheric BOV's will only 'smoke' and run 'rich' when they need to "Blow off" extra air. (And that's to do with changes in Load)
Just tighten it, make it work properly.
That 'Extra' fuel coming through the exhaust would have just gotten burnt by an unloaded engine anyway. Which will not effect your economy. Really, its not extra fuel, because it was going to get injected anyway.
(On a side note, AT A GUESS, because the princeples of induction may counter this - if you plumb back a bov in front of the AFM, you will read that portion of air twice, and then there will be 'extra' fuel injected.)
Your smoke on load is a sign the BOV may not be operating correctly.
I think im just repeating what people have said ?
However there are some myth's being said too.