Do you think B.B. King's guitar sound is anything like Stevie Ray Vaughan's? They're both labeled as "blues".
Do you think Jimmy Page's guitar sound is anything like Brian May's? They're both labeled as "classic rock".
Do you think Randy Rhoads's guitar sound is anything like Dimebag Darrell's? They're both labeled as "metal".
Forget about comparing different guitarists. How about Judas Priest's guitars in Rocka Rolla vs Judas Priest's guitars in Painkiller?
I guess you see where I'm going. Guitar sound is a HUGE subject. There are no hard and fast rules. Everything depends on what you want to hear.
Having said that, there are stereotypical guitar sounds for certain genres. I'll take the risk of sounding stupid and try to give my own recipes:
For blues, I prefer a gain level where you can distort when hitting hard and clean when touching softly. Try to get as much of your distortion as possible from your power amp (volume) instead of your preamp (gain). Set your bass so that it doesn't "boom" when you play the lowest strings. Set your trebles to a level where it doesn't scratch your ears and set your mids high enough to cut through the mix without being overly nasal. No one can say the exact levels like B:4,M:7,T:5 because it depends on your guitar, strings, amp, cab, mic and room.
For classic rock à la Led Zeppelin, add some more power amp gain and if it sounds too nasal cut some mids or add some bass and treble. Nothing drastic though, don't go very far from the blues sound, just make it a little more aggressive.
For classic metal à la Judas Priest, add some more bass and treble. Add some preamp gain but still try to get as much distortion from the power amp as you can.
For post-Metallica metal add some bass until it "booms" slightly and some treble without sounding fizzy. Cut some mids but don't overdo it, for most people scooped mids only sound good with palm muting and you won't be palm muting all the time. And get most of your distortion from the preamp instead of the power amp.
For extreme metal, add even more gain and bass. You can cut some more mids here but be sure that you still cut through the mix.