Originally Posted by: Ian HandOK I’ve been thinking about this for quite awhile now: Why does the Pentatonic minor scale or blues scale (which is also a minor scale) sound so good over certain Major chord progressions (e.g. I, IV, V)?[/QUOTE]
I covered that in a few tutorials:

http://www.guitartricks.com/lesson.php?input=9940
http://www.guitartricks.com/lesson.php?input=10019
http://www.guitartricks.com/lesson.php?input=10023

Originally Posted by: Ian Hand
My thoughts on this are that the Pentatonic minor and Blues scale although they both have a flattened 3rd (which makes them minor scales) they do not have a 2nd note. This I think makes them more ambiguous sounding compared to the Natural minor scale, for example, which does have a 2nd note which emphasises the minor nature of the scale.

You have this backwards. The 2nd scale degree is more "ambiguous" than the 3rd. Any scale that includes the 3rd STRONGLY suggests a minor or major "flavor". In fact the 3rd scale degree is frequently (if not always) THE defining note deciding if a scale or mode sounds fundamentally major or minor.
[QUOTE=Ian Hand]Anyone else have any theories why these two minor scales sound good over some Major chords?

There are two reasons.

1. Ornamental function - the minor 3rd and minor 7th from the minor pentatonic scales played in the context of a major scale & chord progression are half-step approach tones. The basic idea is that one targets a note, say the major 3rd, but plays the minor 3rd right before it for an bit of "delayed" or "prolonged" satisfaction. Or, for a bit of "bluesy" dissonance that then gets resolved afterward. This is the origin of the "blue note" idea. The flat five can also function like this.

2. Structural function - the minor 3rd and minor 7th are used as preparation tones leading to resolutions. This is why dominant 7 chords are so frequent in blues and jazz (and even more so in classical music if you know how to look for it!). Making any given chord in a progression a dominant or altered 7 chord so that it can function as the V of the next chord in the progression is a common musical practice. In blues and jazz frequently this is really the only structural thing actually happening.