Alot of the mojo about blues is that quite often (but not always
) you've got a minor melody over a major harmony, so in soloing you can get away with playing just the minor blues scale
over a blues progression. Guitarists do it all the time.
The key point is that when you're playing bass you're usually not playing over the progression - you are the progression! so as many others have said, outlining the chords is the best place to be.
If you're interested in playing "proper" standard bass, then that's as much as you need to start you off and you can look at scales again after you've got the chord thing down.
If what you're interested in is
soloing with a scale bsed approach, then this is how I'd approach it;
You can make a single combined blues scale by combining the minor blues scale (so in E that's E, G, A Bb, B, D E) with the major pentatonic (E, F#, G#, B, C#, E). that get's you the following;
E - Root note
F# - Major Second
G - Minor Third
G# - Major Third
A - Perfect 4th
Bb - b5th ("blue note")
B - 5th
C# - Major 6th
D - Minor 7th
Which you can also think of as the dorian mode with an added major third and b5, or the mixolydian mode with an added minor third and b5
The advantage of thinking in terms of combining the major pentonic and the minor blues (or dorian and mixolydian if you want to get fancy) is that you can switch between and/or combine the major/minor tonality at will.
The big caveat with any scale based approach is that you still need an awaness of the chords, so that you pick notes from the scale that go with the chords you're playing over at any one moment.
As usual YMMV, etc etc. there are many ways of thinking about this sort of stuff and I'm just offering my perspective on what's worked for me in the past. A very important point to remember about blues is that it came into being outside of the remit of standard diatonic western music theory, so trying to explain it interms of western music theory - while possible - is somewhat against the grain. I'm fairly confident Robert Johnson didn't know what a mixolydian scale was, but I think we can agree he could play the blues.
p.s. there's another note that you won't find at any of your frets, or in any of the scales mentioned above. You have to bend the minor third up a bit to find it (it's between the major third and the minor third).
