I looked it up and found that the key shows you what blues to use during solos. But I find songs with a key of C but using A blues. Does this mean the key doesn't show when to use specific blues scales?? I was also wondering how to know how to figure out what blues scale to use.
The key gives you some clues, but ultimately it's completely up to you which blues scale you use when, especially in a jazz context, you simply play some variations starting on different roots and decide wether you like the results, that's to a large extent the crux of jazz. The whole idea is you can play want you want, express yourself.
So there are no rules, but zooming in a little bit there are of course conventions.
The blues scale is not a particularly well defined thing, I'm a well seasoned jazz player but I don't really find myself thinking in terms of a blues scale too much. But if you want to start soloing over a blues progression (let's say a major blues in C, all chords dominant 7ths) then using a blues scale starting on the root of C is indeed a good place to start. You have to be aware that if you are playing the C blues scale (taking it here as a minor pentatonic with added b5) then the b3 and b5 are going to be particularly 'clashy', but thats the point, they are the blue notes. It might be useful to play for a while with this scale and listen hard to how all of the notes in that scale sound over each of the chords in the blues progression. Individual notes from the scale will sound different when played over the different chords in the blues progression.
Once you feel you have gotten a handle on the blues scale built on just the tonic of the key for the entire progression you can maybe try using the blues scale based on the tonic of each chord in the progression (ie for bars 5 and 6 in a C blues use the F blues scale). This will give you another sound palette to work with, you can stick to C blues for the first time though, then move around the blues scale for the second time through. These are ways you can help develop a solo and create different melodic ideas as the solo progresses.
Moving further you can, as in your example, play the A blues on a C blues progression. This will work fine as A is the relative minor of C and the blues scale has a minor 3rd (the b3rd of the A blues scale is the tonic of the key you are in) and the A blues scale has a b5th (which is the b3rd to the I chord of the key you are in, which is a blue note, a note present in the C blues scale too)
The A blues sale played over the tonic chord in a C blues will highlight, compared to the C tonic;
A - the 6th
C - the tonic
D - the 9th
Eb - the b3 blue note
E - the 3rd
G - the 5th
I'll leave it up to you as an exercise to determine which intervals an A blues scale will highlight over the IV chord in a C blues, the F!
So you can see the A blues scale (or one of the scales often defined as a blues scale) contains no notes considered dissonant apart from the notes commonly considered 'blue notes' which are desired.
So hopefully you can see you could pick any blues scale, or any other scale for that matter (pentatonics are a common option), work out how they lie over your given piece and learn how they sound, for use in your solos. Hopefully you can also see they are all sounds that require a bit of learning to be able to use satisfactorily, as some notes in the scale will be dissonant and seek resolution, while others will be consonant.
Outside of blues and more into jazz this concept is key to finding sounds you like to use and can call upon in your solos. For example you can play pentatonic scales with various different roots over a given chord and evaluate which notes they highlight, and how many dissonant notes they have to be aware of. Some sounds you may discard, others you may like and use regularly, this is what is colloquially called your jazz 'vocabulary'. Note there is a lot more to a jazz vocabulary than just blues and pentatonics, but they're a part of it.
Lastly, don't get too stuck on 'this' tells you 'this'. It's true sticking to simple forms on clear tonics is good when getting started, but you might hear someone play a Db pentatonic over a C major chord and make it sound fantastic, and there may not be a lot of 'theory' there, just musicality exciting the listener by creating a coherent line outside of the expected before satisfactorily coming back home to the c major chord, this is the fun stuff!
I like this question.
First, C Major and A minor share the exact same notes.
Why? The A minor scale is the C major scale but starting and ending on the A note, instead of the C note.
The A blues scale is the A minor pentatonic scale, plus the Eb note: The five note version of the scale (Pentatonic) plus the Eb.
The addition of the Eb note allows a tune in the key of C major to have some C minor effects (ie: E is flat in C minor).
Because of this, the A blues allows us to use the Eb note to draw some C minor characteristics into the tune.
Eventually, we want to not think about this in terms of scales, but rather “colors” and “ideas”.
Thinking purely in scale form limits us to a scale box (I think all of us knows what this might mean: we are looking at a scale shape to provide tasteful notes to play: the good buttons to push vs. the bad buttons to push).
Our job as guitarists and artists is to experiment how the Eb note might intermingle with the other chords in the tune: if it sounds bad to you, don’t use it.
In time, this will incorporate itself into your vocabulary per key (ie, if the tune is in G Major you would use the E blues scale- or e minor pentatonic with the addition of a Bb).
Remember- jazz is an artists game. Yes, we have theory, but discovery happens through experimentation and experience. It’s ok to not have a music theoretical idea at first as to why something sounds good to you: you can always figure that out later.