I am watching a tutorial and he is playing C Blues and he is playing in the bass C and G and C and A. Now the C blues scale is C Eb F Gb G Bb. So how is he playing that A in the bass and it sounds good? I know you can play notes outside of the scale but for example if you were playing in c major and you played c# it would sound way out of place. So when playing the blues, why do people play so many notes out of the scale and it still sounds like its not out of place?
The Blues is a strange term. It includes minor and major Blues. Those notes that are involved in both comprise nearly all of the notes available from the whole gamut of chromatic notes.
Let's be more specific. C major blues notes are C D E♭ E♮ G A , while C minor blues are C E♭ F G♭,G, B♭.
That leaves just a few notes - namely C♯,A♭ and B that are 'foreign'.
So, that means nearly every note in existence is usable in blues. Leave out the 3 mentioned - where's the problem?
Anyway - where's the 'rule' stating "thou shalt not play any notes which belongeth not to the key in which thou playeth"? That does seem to be a 'rule' that even if not taught (sure it is...) then gets absorbed.
That apart, when playing in key C, the F chord gets included at least a couple of times. That F chord has an A note as its 3rd. That means note A fits rather well at those points, beit played by bass, lead, or in the chord that someone else will play.
And, the B note I mentioned earlier comes from the G chord. We're only left with the 'bad' C♯...
What does it mean to "play the blues" and what is "the scale" or key of the Blues?
The blues changes are typically, but not exclusively comprised of all 7th chords on the I, IV, and V. So in the key of C that would be C7, F7 and G7.
The fact is you could play C mixolydian, F mixolydian, and G mixolydian changing when the chords change. Some people do this and it sounds good. As it turns out the "blues scale" is already out of tune relative to these chords. For example you are playing Eb (the minor 3rd) over C7 which has and E in it. The combination of the two notes sounds dissonant (out of tune, or conflicting) but in a way that is musical. The A works because it is in tune, it belongs to the C chord (it is a proper note in the C mixolydian and major scale, and a few others). It also works over F7 as the 3rd and over G7 as the 9th. Another "out" note is the b5 in the Blues scale, the Gb. This does NOT belong to any of the chords but works. Lastly you point out the C# which is enharmonic to a Db or a b9. One of the things that helps with these out notes is understanding the idea of a leading tone. In the major scale the 7th degree naturally moves up to the 8th (the root of the key). The 4th naturally moves down to the 3rd. In all the cases where players are using "out notes" pay attention to whether the move it to an "in note". If this is happening then the player really understands movement and voice leading and is creating dissonance and resolving it.
The b3 and the b5 are very common in the blues and really make it "Blue". I believe that the b5 is referred to as "the Blue note" but I forget. The juxtaposition of these two specific tones over the 7th chords creates a very deep almost sad feeling that is the hallmark of blues. As for all the others? Play the chords and you will see that they contain a lot of chromatic movement. It is likely that players are following this natural chromatic movement in the chord movement from one to the other.
In short it's not so much that any note works but how you use it that makes it work.
"Blues" as a musical form consists of several different chords. The simplest version of a C blues would include C, F, and G chords. The Blues Scale is a scale of convenience: its notes generally sound good over the entire song, even while the chords are changing. But a more experienced musician can choose any note, in or outside of that particular scale, to either match or contrast with the chord being played.
So the A you're hearing could occur during a F chord, which itself includes the note A. But A can also fit with a C or G chord as a more contrasting note than when used with the F chord.
There are many different scales musicians use with the blues, depending on the overall sound they want. In addition to the Blue Scale, Pentatonic Scales (of which the Blues Scale is a close relative) are very common.
Here's one video demonstrating the use of Pentatonic Scales with blues.
The Wikipedia article on 12-bar blues might also provide a useful starting point to better understand blues from the perspective of harmony, scales, and "how it all works."
What this sounds to me is that the player's rocking between the fifth (G) and the sixth (A), which is the key to getting a driving rhythm. This is the technique originated I think from boogie woogie taken by the early rock'n'roll guitarists.
Unlike traditional harmony, you don't find the notes of the chords from the notes of the the scale. And the melody notes don't have to be the notes of the blues scale. Depending on the player, the mood and the song, everything from the major second to the city is fair game, not to mention the blue third somewhere between the major and minor. Granted, I hear that freedom a lot more in pre-electric blues, but it can work if you make it.
It's all more open than you think.