the turning of the blind boy ever so slightly.

Similes are comparisons of one thing to another, as are metaphors. In each case, there will be one attribute that the two things have in common, while little else will be in common.  Most of us are taught to distinguish a simile from a metaphor by looking for "as" or "like"' And in much writing, the comparison is being made in the same sentence, for example, as these are:

Her smile was like sunshine.

His eyes glittered as brightly as diamonds.

But there are other ways to present a simile, and one part of the simile can be in one sentence, with the second part of the simile in a completely different sentence.

In this passage, the "unease" in the mind of this person is the first part of the comparison, in "his head," which the passage uses twice.  The second part of the comparison is the "blind boy."  "As if" is our signal that this is a simile.  To paraphrase the simile, I could say this:

The unease in his mind was like a blind boy that someone had turned so gently that the blind boy did not even realize he had been set off in another direction. 

This is talking about the kind of moment in which your brain is percolating a new and different idea, but it is happening so gradually that you have only the faintest hint of a change. It is a largely subconscious process, enough to cause some mental discomfort perhaps, but not enough for you to understand what is happening.  An example of this might be a scary movie in which your subconscious mind registers a shadow at the window, but this is not enough yet to make you understand you are anticipating a scary moment in the movie, just enough possibly to make you shift a bit in your seat.