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Yeah, Blues uses the imaginatively named Blues Scale, which is based off the minor rather than the Major that the Harp is tuned to. Potential wall of text incoming
The pentatonic Blues scale consists of (in terms of scale degrees): 1, 3b, 4, 5, 7b, (8) - ie a C Blues scale consists of these notes: http://i.imgur.com/hX7KQEO.png
Other popular notes (commonly known as 'Blue notes') are the 4#/5b (which would be F#/Gb in the above example) and the 6 (which would be A in the example above). These notes add colour to the scale.
So, now that we know that we want C, Eb, F, (Gb), G, (A) and Bb to play a C blues, we need to work out what harp would give us the most of those notes.
If we take a look at the harp, we notice that blowing holes 1-4 give us the tonic major chord (1-3-5-8) and drawing holes 1-4 gives us a dominant second inversion chord (2-5-7-2). Holes 4-7 complete the tonic major scale and then holes 8-10 give us the I chord and the ii chord (I'm not entirely sure about octaves but see here: http://imgur.com/a/kH0Q4)