I built one with vol/blend/tone once. Once. I quickly replaced it with vol/vol/tone. It seemed to me that with the blend in the center I was only getting 50% of each pickup, as opposed to 100% of each, and then having the blend roll back each pickup as it moved from center. Please correct me if I am wrong, but it seemed like the output was greatly reduced. I always liked that concept, but it seems the only way I could get it was with an active system. Maybe it was just my choice of blend pots?
On a jazz bass with single coil pickups, when the volume levels are the same level, they work together to cancel hum like two halves of a humbucker. But when one pickup is favored, the bass is susceptible to hum. It's not consistent (some basses get it worse than others), so it's not easily predictable, but it sounds like your bass player's bass is one of those.