Originally Posted by: dvenetianI agree with C Schlegel's post, which points out that this statement is literally the other way around. Being that the scale emphasizes it's determining factor from it's origin and resolves, makes the progression less ambiguous. Ambiguity would leave the progression undefined, or without an obvious definition to the genre. In other words, the sound could be interpreted in ways other than the Blues.
Here's some examples of ambiguous statements.

Walk into a restaurant and the sign reads, "Please wait for Hostess to be seated". A Rather vague way to interpret what the sign actually means.

And another one, "Why did I get a ticket on my car when the sign says",
"Fine to Park Here"?????

More ambiguous tones can be interpreted in more than one way. How you play the progression, leads the Ear to indentifying the statement being made.

Music is a language. We indentify "M" as Major and "m" as minor. In the following statement, "I need to polish the Polish furniture", the minor will resolve to the Major. Vice Versa just wouldn't sound right.
The range of a gifted Vocalist seem to just flow with resolution by accenting their vocal chords with identity.

My Brain hurts...........................

If you play a pentatonic scale over a major chord, the simple fact is that that scale IS more ambiguous than a full-on Major scale. So, yes, if you want to talk about a pentatonic scale relative to itself---you're right, it's not ambiguous because pentonic progressions do and can resolve.

This isn't the statement, at least as I have interpreted it, the question was---why do some minor scales sound good over major chords? Is it because they are more ambiguous?

Now, if you're talking about a blues scale relative to a major chord with respect to the DEFINING intervals of a major chord---I believe that the blues scale is more ambiguous. The third and the root are the most important notes in a major chord. In the blues scale, the root is clearly defined, but the third really isn't the main focus---it's all about the diminished fifth. Just try it, omit the third of a blues scale and work your way around it, I bet it still sounds pretty gosh darn bluesy.
The blues scale really doesn't have a third, it has I and then an augmented second.

So you are right, the blues scale is not ambiguous per say on its own, but in terms of what intervals are important and its relativity to the traditional major scale---it is more ambiguous. The minor third is not necessary to create a bluesy feel--it just isn't. And we've already talked about how a dominant seventh is perfectly fine over a triad major chord. Those two notes are king in blues.

Of course....if you are playing F# blues over A major I can tell you why it sounds good---you're probably implying a major 6th chord---which does sound nice. Not sure though.

Again, you guys can disagree as you see fit--music theorists disagree all the time about things. Hope I elucidated what I mean by ambiguous though. When you are putting a blues scale in a soundscape of major, there is more ambiguity around the central 'strong' notes of a major chord. Just more my school of thinking and where I come from.