Some suggestions, if you're serious about getting a good blues sound you'll take one of them:
Germanium Fuzz Face of some sort with your guitar volume knob backed off.
or
Tube Screamer TS 808
or
Boss BD 2 Blues Driver
or
Crowther Hot Cake
or
All of the Above
The main thing for a good blues sound is to turn your amp on its clean channel up very loud (only if its a tube amp). Turning the clean channel up loud adds just a touch of natural overdrive to the sound, put one of these above pedals in front of it with their level knob up high and very little drive and this will push the amp into a great natural saturation, that's the key to a good blues tone.