No, not muddy at all, not even the neck pickup.
As for dark, depends what you mean. You can get a great Black Sabbath sound (hence the name), and I'd call that pretty dark :twisted: I certainly wouldn't call them bright sounding.
They'll do smooth 'For the Love of God' type stuff, down and dirty thrash metal, clean up nicely if you want them to, and can even be made to do some raunchy classic rock (though if that's your bag there are better options).
Basically all the power you could ever want plus a bit of extra flexability for your calmer moments :)