when light from the sun enters our atmosphere it collides with molecules in the air. The blue part of the light gets scattered more than the other parts during these collisions and thus makes the sky appear to our eyes as blue. If the light from the sun took a straight path down to our eyes with no scattering or absorption in the atmosphere, the sky would in fact look much as it does at night in the day time.
The Earth’s atmospehere is full of gases – from previous questions you know that light emitts blue when it bangs into gases. The availability of gases changes as you leave the Earth’s atmosphere, so light passes through without banging into anything. 🙂
The only thing I have to add is that there’s this really cool image called ‘the thin blue line’… it shows the change from the blue of the sky to the black of space. It makes you realise how small our atmosphere is. It really makes me want to protect it! 🙂