A blend pot on a jazz replaces one volume pot, and is a pan pot between the pickups. Does that make more sense? You have a center detent and as you turn it each way you get more of one pickup and less of the other, but the level doesn't change. The volume is in effect a master volume and the tone is a master tone.
Thanks for your kind words Jesse. I should have added that you also have a lot of control over tone with your right hand - where you pick, closer to the bridge, the neck, or the middle. Whether you pick with your fingers, a pick, one finger, two fingers, pluck with your thumb or slap with your thumb or figertips. I also think many people don't realize that how strong your left hand is has a lot to do with tone as well - if you you're really strong and have good left-hand technique your tone and your pitch is clearer and more consistent. Since it's actually the fret and not your finger that stops the string on a fretted bass how hard you press down on the string behind the fret can vary the pitch subtly but audibly. Try it. Consistency is the most important thing for good pitch, but having a really strong left hand gives clearer tone.
When I was learning to play there were very few alternatives to Fenders, and they were all passive. So you really had to deal with this stuff. Once you get used to having that degree of control under your hands you don't, or rather I don't, like to give it up to devices. I hate being over-compressed. I know how to keep my level consistent and if it's not consistent it's because that's the musical requirement the way I perceive it. I was doing a live TV recording and the engineer from a huge, well-known audio truck came in and gave me a very patronizing lecture about keeping my dynamics even. I explained to him that I was playing with an ensemble in a hall, that my dynamics had to be tailored to fit the ensemble, and he would have to capture the dynamics as best he could. I never heard the tracks.