Its highly unlikely something is wrong with the pickup.
Check the wiring again; it MAY be that because BK use different colour coding you've got it wired split (unlikely I'll grant you) or out of phase (more likely, but thats a BIG difference in tone).
Also, how close is the pickup to the strings? If its far away, try getting it about 2.5-3mm and see if that takes you where you want it to go. BKs are abnormally sensitive to pickup height, and big changes can happen just by moving them round.
If that doesnt work, try doing that and flipping it round 180 degrees so the screw poles are closer to the neck. This will increase its low end and low mid output.
The other possibility is that the MM just doesnt agree with that guitar. I've had one MM, the same MM (I've owned a few) sound very different in different guitars (and I mean VERY different; thick, powerfull adn meaty through to thin, gay and tinny, depending purely on the guitar). Given superstrat tendency toward brightness you may have suffered a dissagreeable pickup/guitar mix. Its not the pickups fault or the guitars, they just sometimes dont blend together well.
If this is the case then try a 250k pot (assuming youre using a 500, which is probable), plus any combination of the above, if need be.
And, welcome.