It'll only be hum-cancelling if the BKP is reverse wound, reverse polarity (RWRP) as compared with the neck pickup. I don't know if BKP wind their P-90s in the same way as Duncans or not.
Turning the pickup round wouldn't make any difference.
If you find it's NOT hum-cancelling, you could move the Duncan bridge pickup to the neck position. That should work.