I sometimes read about the major and minor blues scales. Now, this makes me a bit confused. Are there really two kinds of blues scales? I was told that in a 12 bar blues you play the I chord for 4 bars. Now, a piano teacher told me that in some blues the IV chord could be played in the second bar. What kind of blues would that be?
What you probably mean by minor and major blues scales are the two following scales (with root C):
C Eb F Gb G Bb (minor blues)
C D Eb E G A (major blues)
These are just the minor and major pentatonic scales with one note added. The minor pentatonic scale gets a b5 (Gb), and the major pentatonic scale gets a b3 (Eb), both to make those pentatonic scales sound bluesier. Note that both scales are modes of each other, i.e. the C minor blues scale and the Eb major blue scale have the same notes.
A standard blues uses dominant seventh chords on the I, IV, and V. For such a blues both scales can be used. The minor pentatonic scale with the added b5 (which I and many other people refer to as the blues scale) works well over all 3 chords. What some people call the major blues scale works well over the I chord, but not all of its notes work so well over the other two chords, so you have to be a bit more careful with that scale. [Note that I refer to the use of one scale with the root of the I chord over all three chords. If you change the root of the scale with every chord change (e.g., C major blues over C7, F major blues over F7, and G major blues over G7), then there are no notes that must be avoided.]
A totally different thing is a minor blues, which can be recognized by the fact that at least the I and the IV chord are minor seventh chords. In this case you obviously can't use the major pentatonic (or blues) scale, but only the (minor) blues scale.
The IV chord in the second bar, which your teacher referred to, is just a variation of the standard 12-bar blues (with dominant seventh chords), and it does not really influence the choice of scales, other than that you get a chance to sound horrible already in the second bar if you're not careful with the major blues scale.
As a final note, there are many scales that can be played over a standard 12-bar blues (if you're not afraid of sounding a bit unconventional). Guitar player Oz Noy gave a nice (albeit slightly artificial) example at a clinic: read about it and have a listen!
"Now, a piano teacher told me that in some blues the IV chord could be played in the second bar. What kind of blues would that be?"
I'm not sure whether it's local terminology but among the jam sessions I frequent, this is called a fast-change blues. You play the IV chord in the second bar then return to I, then go to Iv again after a total of 4 bars as you would in the usual blues progression.
Essentially you're just putting a IV chord in for the second bar, just for that bar, just for a bit of effect. An example would be "Drink that bottle down" by the Stray Cats
Blues scales don't exist, they are licks.
Ex chord I7 (ex in C) : can compose any blueslick with combinations of ...
Descending: Notes triad mixed with descending (doubled) appoggiaturas : C Bes A G // G Fis F E // E Es D C Sadness - Downwards ( rem. Bes here is not a chord note!)
Ascending: Notes triad mixed with +ascending appoggiaturas: C D Dis E // E F Fis G // G A C Hope - Pentatonic
Same principle for other degrees(Chordnote=>appoggiaturas=horizontal motion=>chordnote) Appoggiaturas create also the possibility tu use in the chord nested harmonic functionalities."