I don't understand why true sight would be considered a different sense than sight. If you have been blinded, can you use true sight to perceive your surroundings? I don't think so.
In any case, true sight helps you to see invisible creatures, see through illusions, etc. A creature using Hide from Dragons is not invisible, the spell is not an illusion, etc.. So true sight is no help.
The Draconomicon version omits the sentence (found in the Hide from Animals version) that mentions tremorsense, blindsense, etc. So really it only mentions seeing, hearing and smelling. I don't think that means that a dragon can use its innate blindsense. That is ruled out by the phrase "the dragon simply acts as though the warded creatures are not there." I venture to say that the line was omitted because it is redundant.
If it is true that acting as though the warded creatures were not there means that blindsense is nullified, then other senses would also be nullified; even ones not explicitly mentioned. So acquiring blindsight from a spell would not help.
Having non-dragon guards is a good idea, though. And alarm spells would alert the dragon that something was amiss. He couldn't perceive the warded characters, but he could blanket the area with his breath weapon.
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One other question: A no-save Hide from Devils- what level should it be? Does 5th sound fair?