key signatures

I know that in the major scale the key signatures are: C, none; G #; D ##; A ###, and so on but what happens with the blues scale and harmonic minor scale? are the key signatures the same? if I see a song with one sharp but I know the guitar player of that song plays blues scale, what's the key of the song? is it G?

Blues shows that traditional theory can't explain everything. There are a couple of approaches. The blues scale can be used from the relative minor of the key, so that in the key of C, with no sharps, you will have an Amin blues scale. This is more of a rock thing. It is more common in blues to play a C blues in the key of C, with the chords being C7 F7, G7. Of course, if this were written out there would be a hell of a lot of accidentals, becuase the C blues scale has a minor third, a flat 5th, and a dominant 7. I think I might not be helping very much so i will stop in the hope that someone can more coherantly tell you.

For the most part, blues is a free form style. Theory and all that doesn't really matter that much in blues. You just play what sounds good.

Just in case you were wondering - The Blues scale has a sharp 4th in it. Harmonic minor has a sharp 7th.

Key signatures only tell you what major or minor scale the song is based off of.

For minor, the key signature gives you the relative minor form only (Aeolian). There is no specific key signature for harmonic or melodic minor, you have to use your ear to see if either of them apply, or if you have the music look for the dominant five chord.

For blues the key signature tells you what major scale the 1 4 5 progression is based off of.

I think what your asking is how to apply blues scales over major progressions ? To do this & stay in key play a blues scale a minor 3rd down (3 frets) from the major key. You can also, in most circumstances, play a blues scale off the root of the major key for a bluesy sound. Good Luck