I did debugging for this thing, and I found that if I set BLUEBIRD_DEBUG=0, then when your code calls !!process.env["BLUEBIRD_DEBUG"], that expression is true, because !! applied to a string that contains "0" is true.
There was a breaking change in one of the recent releases, because I've been using BLUEBIRD_DEBUG=0 for a while, as was suggested to me here a while ago, and it worked. It stopped recently, not sure when exactly, I just noticed by accident that my tests on Travis where I used BLUEBIRD_DEBUG=0 started showing huge slow-down, so I went investigating till found the culprit.