Yeah, blend can be tricky with passive basses. Imo two volumes work better. What I'd probably try are two 500k linear taper volume pots. One disadvantage of 500k pots is that you have less granular control; linear taper might help counter that (some have confirmed this, but I never actually tried it for blending two pickups).
Another thing some people do is have a master volume, and then a volume for bridge pickup, which allows you to blend in the bridge pickup to taste (But you cant solo it), and you keep the master volume function.
for the tone control, you could try a 500k pot (audio taper!), but some feel like you lose too much control vs 250k pots. I didn't have that problem. Another option that combines the two is a 250k no-load pot, which gives you the function of the 250k pot but is out of the circuit when fully open and this gives you the brightness of a 500k pot (even brighter).
For your pickup choices, the traditional approach would be to put the hotter pickup (I think that would be DiMarzio) in the bridge position, as they will blend better. DiMarzio at the neck would overpower the other pickup. BUT, it may actually work well for what you want so try it. You could also add a series/parallel switch for the DiMarzio - it would lose some of the oomph in parallel.
