In the notes prior to the sheet music, it is said that an accidental note only lasts for one bar. Here in bar 3 an accidental note has been applied to the C in bar 3. In such a case why has the sharp note been added to the C in bar 4 (the note for 'broom') also? Wouldn't that auromatically be a C# as the accidental note's effect ends in the 3rd bar?
If you here the vocals track for this song, the note for 'broom' sounds like a C# also.
They call it a "courtesy accidental". In your particular case, the C♯ is written in because of the C♮ that occured recently before it.
Sometimes written in parentheses, they exist to remind performers of an accidental that applies. These are written usually where a note is tied across a bar line, or otherwise the note it emphasises is in a position to be forgotten. 
Examples of the same above, though there's plenty of occasions to use them.
There is absolutely no need for that #. It's just a friendly reminder that the C in that bar is actually C#, even though any C on that line was going to be C# anyway; the previous C# cancelled down to a Cnat. being in a different octave, therefore not affecting that middle C note anyway. That apart, the previous barline cancelled the higher C.
Often the courtesy accidental is in brackets, which makes more sense - and says 'yes, I know there's strictly no need for this, but...' But putting it in like a proper accidental, as here, is confusing - as you found.
To actually answer the header question: In Blues, there are basically 3 flattened notes that get used often - 3rd, 5th and 7th. The m3 over (under?!) a major chord is commonplace, sometimes inflected upwards. The b5 produces a triton, very dissonant. The m7 helps towards the dominant 7th sound of the main chords.
