Can I use blocking between floor joists to support off-center jack studs in a load-bearing wall?
That should be just fine. A few points for clarity:


Use substantial (joist-size) lumber. These are essentially joists themselves. I wouldn't go smaller than 2x6. 
Consider using joist hangers or lag screws. Sheer nails don't meet modern code in a load-bearing situation like this--either at the ends or into the existing beam. 
You might double them to give better bearing. Double joist hangers are easy to come by. 
You could drop these new joists 1-1/2" and lay in a wide plate on top of them, and under the wall plate, to give yourself flooring/subfloor support around the wall plate.