Welcome to the Jag SS world!

Hope this answers your blend question. I have a Squier Precision Special (34" P with jazz neck and p/j pup) that is wired with V/B/T. I really like the master volume control, it makes life simpler for me with no loss in tonal flexibility/quality. I find the blend makes it faster and easier to switch between pickups, turning one knob instead of two. My Jag SS controls are stock and although they work fine, at times I wish for the master volume / blend controls of my P special. I can't add any details about what specific parts were used, I took the bass to a local repair guy I trust and he supplied parts and labor (about $35 in 2002). I think there is some info on the internet (check StewMac) seems I remember 2 different types of passive blend pots being available. The one I have has a center detente where both pickups are on 100%. As you rotate the knob towards a pickup and away from center (lets say towards the bridge) that pickup (bridge) remains at 100% and the other pickup (neck) will decrease in volume. At the extremes you get 100% of one pickup and 0% of the other pickup. This is the type blend control function I'd recommend (
a thread on TalkBass). Hope this helps. Here is a V/B/T diagram from [Invalid or Expired Link Removed] and here is
a thread on TalkBass about wiring a blend control. IIRC you treat the P bass pickups as a single unit like the neck J pickup when wiring (
P/J blend wiring diagram). Here
Link Removed (I have purchased a complete wiring harness for a P bass from them, great customer service and parts quality, came with easy to understand diagram). Hope this helps.
The P Special has Duncan SPB1 (P) and STK1B (J) pickups. I think these are a major improvement, providing a great vintage tone, over the original pups which were pretty bad. IME I'm not sure the cost of replacing the stock Jag SS pickups, which are far superior to the one in the P Special (especially the P), with these specific Duncan pickups is worth the addition money. The STK1B is noiseless when soloed which is nice but a little weak. I really like the tone of the stock Jag SS P pickup which is soloed 80% of the time.