It would depend on what kind of a blues sound you are after too. Texas blues, Missisppi blues, Delta, bebop, etc. And what kind of guitar you are using. I would agree that the JC-120 wouldn't be my first choice for going after a bluesy tone, but if you are using a hollowbody and going for a Duke Robilard, or T-Bone Walker kind of tone you might be able to aproximate something close by turning the treble down to 4-5, the mids around 6, and the bass from 3-5....but the biggest factor is gonna be your hands.

I would suggest selling the JC120, and getting a small 30 watt class A tube amp, or 50 watt class AB. Part of a good blues sound is using a good sounding tube amp just at the edge of overdrive.